Troll Warlord is a unique hero in that he can instantly switch between melee and ranged, with a separate spell corresponding to each one. Outside of that, he is all about attack speed. Fervor increases attack speed on the target by 20 with each subsequent hit, and Battle Trance increases the attack speed of himself and allies for seven seconds. Thus, he has a lot of presence at all stages of the game, so whether you want to viably run a five carry strat, or play him because he's a troll, Troll Warlord will not disappoint.
Beserker Rage is your form switch skill. You default in ranged form, but it is pretty much an instant toggle, and melee form is stronger for a few reasons. You gain more movespeed, damage, armor, 100 more HP, a lower Base Attack Time, and have a 10% chance to bash. However, the scaling is very poor (bash chance doesn't increase, only the duration and damage by a bit, and only a 10 movespeed increase at level 4), so we max this out last.
This item plays a part in Troll's strength in lane. His base damage is good enough to last hit without melee form, so if there is a lot of harass you still have a chance to farm. Also, ranged form means you can harass on your own.
Whirling Axes has two spells depending of what form you are in. They do not share cooldowns, so you can use one freely without having to worry about not having the other in a situation.
The ranged form is a decent range, small AoE slow. In lane it's pretty good for initiating, especially because if there's no follow up you hardly waste any mana. The melee form is a low manacost AoE nuke that causes attacks to have a miss chance--125 damage at level 1. This makes for a pretty scary lane if your support has some kind of stun or slow. It's also pretty good for farming and scales the best with levels, so we max this first.
Note: Whirling Axes is very important if you want to manfight, because the miss chance means you will pretty much win.
Fervor is part one of your attack speed steroid. As you attack a target more and more, you build up up to five stacks (level 4), and each stack gives you 20 more attack speed on the target. If you switch, the stacks reset. I typically get one point at level three to help with buttering creeps, but it is fine to get two points in Whirling Axes first for more nukage.
At level 3, this ability gives you and your allies 180 extra attack speed for 7 seconds (5 carry strats all day please). Before 6.78, this lasted for 10 seconds and had a 20 second cooldown...balanced. Now, though, it's still very spammable. If your team is ganking or taking a small fight without you, it is still completely recommended to use Battle Trance. Did I mention it only costs 75 mana?
[*] Optional change from listed build: Whirling Axes at level 2 and 3, then Fervor at 4. Just don't get extra points is Berserker Rage until after everything else.
Alright, the ordering of these section is a bit weird, but I think this is the best way since I will probably make reference to the skills (Whirling Axes mainly) in this section.
Optimal Case: Safe Lane Farm with Stuns / Nukes
As with any carry, the best case is to be fed. Luckily, Troll Warlord "eats" pretty well. Level 1 first blood is a bit hard, since your main nuke is melee Whirling Axes, which you won't have. Therefore, I usually tell one support to prepare a stack and have the other sit in lane. In a more common dual lane, your support can stay since you will hit level 2 pretty fast.
If you can harass the opponent a bit with ranged form, all the better. Whatever the case, be aggressive. Don't dive, but throw out your slow and your nuke, smack with melee and hope for a bash, and then switch to ranged if they get too far. As long as you attack them not at their tower, you will most likely get a kill. From there, if you can still be aggressive do so. However, if they play safe, don't go too deep trying to get kills. From here, you can just put pressure on the tower.
After the tier 1 falls, which is pretty fast because of Battle Trace and that early point in Fervor, I'll look at the other lanes. In a passive game, where mid and the offlane are given room to farm, I'll generally farm jungle and push out the safe lane when creeps are at my tower. Whirling Axes is great for this because of it's low manacost and decent damage.
In a more aggressive situation, where maybe a couple of enemies are sitting mid or on our offlane, I will transition to push other towers. Sometimes this means the tier two, but generally it's the other tier 1s. These early towers secure a gold advantage and map control, and help to win midgame.
I do not recommend picking Troll with going mid in mind. This lane is more of a adaptation lane if someone picks a hard carry who needs safe farm after you. Since this is not a mid Troll guide, I won't write much on it.
I basically go for the same items, and farm with melee Whirling Axes or ranged attacks. Bottle is optional; I would say it you enemy has the potential to deny you heavily, such as Queen of Pain, get a Bottle so you can spam Axes for last hits. Otherwise, just continue with your item progression. Either push the tower if the opponent leaves, or go gank a sidelane and push the tower.
See items (BKB in particular) for post-laning play.
Standard three branches and regen start. The Circlet is for the Bracer into Drum.
Alternate Starting / Early Items
I typically don't build Aquila anymore, though Ring of Protection is viable for Basi into Aquila. However, if you anticipate a lot of harass in lane, Ring of Protection into Tranquils really helps you stay in lane and in the jungle.
Normally I get this on all Agility carries with a Quelling blade, but Troll's +15 damage in melee form goes a long way. If the harass is primarily going to be right clicks (IE opponents don't have a low manacost nuke), this item is good as well.
A 12% slow (in melee form) synergizes well with ranged Whirling Axes. However, I usually find OoV unnecessary since if they manage to get away I can still kill them with ranged attacks.
The early items are fairly self explanatory. If you can get the last hit for first blood these will come along very fast. #obviousstatements2013
The extra damage and movespeed go a long way for early game aggression. 2 stacks of Fervor give 10 more attack speed than Treads at the expense of the damage and speed.
I don't see a lot of Troll Warlord players buying Drums, and it is a debatable item. While it is fairly cheap, you could argue Troll needs Helm of the Dominator or a faster BKB. However, with the early towers from this kind of playstyle, your items progression is reasonably fast. You should be able to farm a BKB with the map control from early towers. I just think Drums offer too much of what Troll wants to skip this item. Strength is great for the HP. Agility and the 3 damage means +12 damage from this item. 9 Int = 117 more mana, which means two more Whirling Axes not counting passive regen. 5% movespeed goes farther the more movespeed a hero has; Troll has 320 (330) in melee form. Only the 5 attack speed is a bit wasted, but not even that much since Troll has a base attack time of 1.55 in melee.
tl;dr: Drums is a very strong item that scales very well into the midgame.
Black King Bar
The only time you don't want to get this item is if the opponent's use stuns arbitrarily instead of focusing you down. That should rarely happen, so you should not skip a BKB in many games. Troll wants to be aggressive early, and killing enemies basically also means killing towers. To kill enemies, you want to ability to jump into a fight immune to magic--Whirling Axes's 60% miss chance will ensure you don't get right clicked down. If you take no magic damage and barely any physical damage, you can wail on anyone safely, and thus probably win the fight. And how convenient, Battle Trance is already off cooldown when you reach their tower. Seriously, buy BKB. It's stats are not even that bad.
Later Items (But Hopefully Not Too Late)
A little damage and some lifesteal that builds into Satanic...'nuff said.
Helm is one item you can pick up before BKB if the game is more passive and you can get by farming jungle and split pushing. It is the preferred lifesteal because it builds into Satanic, which is OP when you have +300 attack speed.
25% lifesteal is no joke when you attack like 3 times a second. Throw in the active and you basically won't die. This item is definitely superior to Heart because Troll doesn't really need a different UAM and Satanic pays off more when you attack so fast. Note that Heart is still an option if you really need that much tank, but hopefully you won't.
Manta is generally the go to item for Agility carries, with the exception of Faceless Void and maybe Juggernaut (the former because illusions can't bash, the latter because Desolator or Aghs are available). Troll's illusion can't bash either, but illusions are always a strong option for pushing, something Troll excels at. If your team already has decent push, you don't exactly need this item, though bonus movespeed will be missed. Also, sometimes a Yasha is enough.
Generally you don't want attack speed items on Troll since he scales so well with just damage and Agility alone, but for Butterfly the attack speeds is more of a bonus. +30 Agility and +30 Damage in one slot alone is enough reason to get this item as the game progresses, and the attack speed and evasion are just added benefits. Plus, the extra evasion means any enemies standing in Whirling Axes without MKB will miss 95% of the time. (I think, the stacking is weird, but since Whirling Axes applies a miss chance, it should be something like that).
Crits are great because of the damage potential; just don't rush it after Phase / Tranquil Boots. The DPS is rather meager until you build up some more damage items and get some more Agility through level gain and stats. It also provides you with no survivability--one stun and you're pretty much dead. Nice DPS.
After you receive the full potential of your attack speed bonus, however, and have life steal on top of that, a couple crits can mean a won fight. It pays off much better if you build it a little later.
I generally don't build this because the opponent is far from Butterfly / Heaven's Halberd. However, if you see such items started, or you are facing Phantom Assassin or Brewmaster, I recommend getting this item after Helm.
If you ever do get this item on Troll, the game should be pretty much over, or approaching the 70 minute mark. It stacks with Lifesteal, which is nice, but doesn't provide as much DPS because of its more rounded stats. It's fairly expensive as well.
It's a shame Troll needs lifesteal and really likes Satanic, because Desolator or Diffusal Blade would be great on him. With Vlads, you get some regen and that lifesteal, at the expense of a potential Satanic.
We don't pick up Drums in this case because we still want BKB most of the time as fast as possible. Desolator really helps is pushing, so it is useful if somehow Troll Warlord ends up being the only real DPS on your team (Battle Trance lets you be greedier and have up to 3 core heroes relatively efficiently). Diffusal is generally only picked up on illusion heroes, but it is an option with Troll's attack speed. The slow is also good for keeping opponents in Whirling Axes.
These are fun builds (I really like Desolator's lasers) that fill situational niches, and if you dominate lane the outcome of the game shouldn't be determined which build you go. Just know that lacking a Satanic late game will really hurt your carry potential.
Troll Warlord is a very strong early and midgame carry who does not rely on 40 minute AFK farm. Because Battle Trance is global, both split pushing and fighting are strong options to gain an advantage. If you like being aggressive as well, Troll Warlord definitely fits that bill.