This is a set of builds for Naga Siren. I love illusion based heroes, and Siren is one of the more versatile ones. Each build gets it's own minisection for you to check out, so consider this an anti-alt-tab guide. They are sourced from the various Dota scenes and my personal experience. The formatting for this guide is matched heavily with my Faceless Void guide, which you can check out if you play him.
Naga Siren is one of the most versatile and outright strongest heros in the game. No matter how you slice her (stats, skills, armor/movespeed, ultimate, winrate) she is a fantastic hero capable of fulfilling any role from 1-5.
As a hero Siren has two areas that need focusing -- serious mana issues and stacking stats/HP for her illusions. Teams build her a lot of different ways, which I'll try to cover in rough order of popularity. Generally, the Chinese teams will build Linken's first and North American teams will go tranquil boots+radiance into hard carry items, with Arcane Boots+ Vanguard into Diffusal Blade as a less common midgame option, while the other scenes build her a ton of different ways. I play a lot of Southeast Asian (Pinoy) Dota, so their top builds are in here as well.
Siren is relatively tanky (2.3 strength gain on an agility carry, 5 starting armor, easy get-out-of-jail-free ulti, illusions to dodge/confuse stuns), a strong pusher/teamfighter with an awesome setup/getaway ulti, and a great laner. She has a very strong single target initiation in the form of Ensnare, which helps secure early kills and makes her an effective Blink/invis/BKB hero counter. She's a hero that not only likes level 16 but has the stats and scaling to take it to 25.
Note that for time calculations this guide assumes around ~350 gold per minute. Early less is expected and later more, but this will help you identify the time constraints you're under for certain builds without having to run them multiple times. Siren can farm multiple camps at once/plus lanes with ease via Mirror Image with some basic attack move and Rip Tide micro, so expect literally insane gold per minute after 25 minutes.
Another important note is that skill and item build are not a one-size deal. If you go mid a bottle would make sense. If you're getting free farm in lane and your team is pushing heavily maybe early image levels should take priority; maybe you need stats early against a trilane or a single mirror image early against a Sven to dodge storm hammer. There isn't a way for me to offer alternative skill builds, but know that you can be successful with many different ones as all of siren's skills are fantastic.
I'm not going to list out skills/whatnot, there's a lot of excellent videos and replays of how to play Siren. An easy way to get better is to watch replays on player perspective -- filter recent games for Naga Siren and very high skill level. I will cover some of the points I don't see people execute consistently, even at higher levels.
This build is very safe if a bit boring and probably outdated. Fast RoH into Vanguard makes you survivable early while the mana boots help solve your mana issues. Your DPS comes from your illusions, which are later augmented with the standard naga (or really illusion hero) pickups.
Vanguard is actually a pretty ****py item. If preferred you can easily make your stout shield into a poor man's shield, save the RoH for a Linkens (or go tranquils or even just keep it a casual RoH), and the vitality booster for your eventual heart. Alternatively, you can grab armlet over vanguard, with the helm of iron will as your first piece.
The vanguard and mana boots give you good HP/mana. You don't have the mana regen to spam images/rip tide every time it's off cooldown to farm/push but you have good teamfight presence with just your mana pool off of arcanes and the HP from the vitality booster component of vanguard.
Naga benefits a ton from and is commonly built towards the same 3 items lategame -- heart, Diffusal Blade, and Manta Style. The diffusal burn is carried by your images while the heart makes you and your illusions crazy tanky for pushing/teamfights. Manta adds more movespeed to naga's already great starting along with stats for your illusions, which you now have more of.
This build rushes Linken's, and was seen dozens of times at the international. Linken's tells the other team that you're going to be farming/pushing constantly and that any attempt to gank you will either be unsuccessful due to Linkens+song or take a ton of resources (3+ heroes) to succeed as it Linken's almost ensures you'll get your song off. Basically it gives you a pretty free license to be out of position split pushing as you just sing tp as soon as they show up and pop your Linkens. If you do notice 3+ heroes miss all you do is leave images to push while you move off map to jungle. Travels are gotten in games where you want to farm/split push incessantly.
The RoH with possible tranquils makes you very hard to harass out of lane/lets you jungle freely, and song makes you hard to gank (even at level 6). The buildup to Linken's isn't that great but isn't bad and as soon as it's done you can just start spamming images down lane or in jungle. In classic Chinese style Hand of Midas can be gotten before you finish Linken's (either after RoH or before if you really forced the longlane solo out).
If you made it to Linken's on either regular boots or tranquils Boots of Travel are good for the farm, push, and chasing power they give as illusions don't get the attack speed from Power Treads only +8 of a stat. Same thing as North American build -- Naga is commonly built towards 3 items lategame -- heart, diffusal, and manta. Linkens and your illusions make it very risky for the other team to try to use single target disables on you in teamfights, but BKB is always an option if you're not getting consistent DPS out during teamfights due to CC.
An increasingly popular inhouse build that employs the trusty Tranquil Boots Soul Ring combo, giving you stupid 10Hp/second average regen, +80 movespeed, with mana on demand to spam Mirror Image/ Rip Tide. After these two sustainability/laning items that will basically allow you to win the lane you rush either Radiance or heart of terrasque. Radiance is if you're looking to farm multiple lanes/camps and concentrate your power late game or heart if you want to push hard early and be stupid tanky. If rushing heart consider early Hand of Midas to get your farm out of control, but it probably isn't worth getting on the radiance build, as it would delay radiance, which is bad considering how weak radiance gets if it's late and how much the burn damage helps you farm anyway.
Pretty easy item build, though by going this you are pretty well dedicated to sitting in lane/jungle farming like a hardcarry for the first ~20 minutes, pushing as song is off cooldown if there's action elsewhere. If your game isn't going as well you can pick up a Vitality Booster before Radiance. If things are going less well or if you're more focused on midgame you can rush heart/manta instead. After you finish your first item you can drop your tranquils for power treads, but it's not required or even always the right choice given the movespeed they offer.
Radiance lets you farm multiple lanes/camps at once, using Soul Ring to solve your mana issues short term. You won't be spamming rip tide to clear camps, rather just abusing radiance burn. Either build you can just shove (attack-move) illusions down lane and it will require a hero there to stop the lane from pushing. You can tank towers with heart, pushing with the creep wave as song is off cooldown. If you went radiance you'll be getting very fat very fast. If manta or heart you'll be forcing more teamfights early.
Originally a terrorblade build I faced in a clanmatch, I've recently seen this exact build on a quite a few agility carries -- Spectre, Anti-Mage, even Juggernaut. I think it was originally inspired by Chaos Knight's build. This build focuses on a weird item for an agility carry -- Armlet of Mordiggian. It actually makes use of three of the most cost efficient items in the game (armlet, Vlad's, drums) to give you a strong midgame with a good lategame transition. Armlet/drums have solid synergy with Siren. Her built-in illusions benefit from armlet health and drum aura -- activate your armlet before you split! Vlads is good for teamfighting early as always, but this build does not solve your mana issues without arcanes on you and preferably at least one other hero.
Early tranquils give imba regen -- consider keeping them even after you make Vlad's as the movespeed and regen is just so good for 525 gold (which you can later sell to make half back on). This build gives a lot of early stats which gives you a pretty strong 12-20 minute timing window to push with team and force teamfights. I get an extra early level of images so I can have them maxed at 10 and not 12.
Manta or heart are the obvious transition out of this, as you'll hopefully be winning teamfights with your Vlad's+Drums aura and armlet illusions. Manta gives you more illusions to benefit from your items while also stacking movespeed and stats for them/you. Heart becomes an awesome endgame item as it lets you leave Armlet on all the time. Diffusal remains relevant as always.
The Malaysian scene does a 4/5 role Siren occasionally. It's pretty viable as she has great (BKB penetrating) disable, laning utility, stacking and pushing with illusions, great farming for a support, and the ability to transition into a carry.
Get a support item, not all 3. As a support you should provide your carry with lane presence, harassing, coordinating for kills, and pulling as needed. You will not have many last hits but get tranquils for sustain.
Get a sage's mask item -- basilius for pushing, medallion for Roshan/ganking, urn for ganking and pushing. Make sure you have flying and wards/dust as needed. I sometimes get two sage's mask items, especially if I'm given space to farm as riptide and mirror image provide a means to turn mana regen into GPM, a luxury for supports.
As you get towards lategame you can get treads/vlads as you start to rightclick more. Diffusal provides some extra utility with the purge and manaburn but you can grab pure support items here instead, though admittedly Vlads+Diffusal is pretty good. If the game goes later and later or if you get fed Halberd, Manta, and Heart all provide utility while shifting you into a more pure carry role.
You have two big concerns:
What role does my team need me to fulfill? and
What's our timing?
If your team is up against a harder carry ( Spectre, Anti-Mage) I would suggest Vlad's armlet or arcane vanguard so you can hopefully push and take an early advantage and avoid taking it lategame. If you're the hardest carry on the map (easily possible, Siren has crazy stats and scaling) Linken's or Radiance are good. Obviously Doom Bringer/ Beastmaster provide an easy incentive to go Linken's. If they're squishy with lots of supports radiance is great.
I can't give you a list of every scenario and every pick, so here's an easy pro/con table for the builds to help you choose.
Early teamfight tankiness and some mana
Less than huge teamfight presence
Not that good at farming or pushing
Doesn't benefit illusions much
Annoying split push as soon as you have some levels in mirror image
Comes partially online fast
Very strong farming ability
Scales well lategame
Makes you a very resource intensive target to gank/stall ala Morphling
Stats for illusions
Only moderate teamfight
Illusions don't get too much
No early HP
Requires at least one other item to become a huge presence in fights
Tranquils+Soul Ring into Rush
Solves early mana issues
Lets you farm freely for radiance/heart/manta
Forces you to sit there farming for major items for the first 20 minutes, with some split pushing when you have song
Early HP and stats
Good teamfight auras
Comes partially online fast
Benefits illusions well
Great transition into diffusal/manta/heart
Armlet requires attention to turn off/on
Does not solve mana problems more than partially
Tight on item slots
Lots of utility -- net, -armor, 10s nuke, song.
Obviously less DPS and farming capability than a carry oriented build
More reliant on allies
Larger burden of knowledge with how to support
So there's the most common/effective playstyles for Naga. If you have any questions/comments/criticisms feel free to post them. At the very least I'd like a comment why if you feel like downvoting :P. If you feel it was worth the read a vote up helps get builds seen more, and I certainly would love to see some more non cookie-cutter Sirens.