Leshrac is a AoE Teamfight hero with a massive amount of pushing power and with a good start has potential to snowball out of control. He can be played as a farmer or a lane support but this guide will be focusing on a farming leshrac.
Split Earth is your only stun and one of your two nukes it has a VERY obvious cast animation and is better used if you have someone else for a set up stun to ensure that you hit yours.
Diabolic Edict is one of your major AoE or single target spells as it does damage to random enemies in the 500 radius around you this spell is the main reson why leshrac is a huge pusher since this can also effect towers.
Lightning Storm is your second nuke that also hits an increasing amount of people as you level it up this can be used in pushing or counter pushing to mow down creep waves or in teamfights to add to your arsinal of AoE abilities.
Pulse Nova is the third of your AoE abilities dealing damage per second for mana per second to everyone in a 450 radius around Leshrac this combined with Diabolic Edict can make you fearing in teamfights.
I prefer to go pretty low on regen to start out with to help you get your bottle quickly but still having enough regen to stay in lane until it arives. I would not sugjest starting off with your bottle as the iron wood branches will help you in last hitting as Leshrac has pretty low base damage.
The bottle is a good item on leshrac because it will help solve your mana problems in the early part of the game and coupled with runes can help you gank side lanes.
Arcane boots are the best boots for leshrac early on as the extra burst of mana they provide can realy help. Bloodstone is amazing on leshrac as it will help you tank up which is a major problem for leshrac and it will give you more mana regen as you get more charges allowing you to keep your ulti on for a longer amount of time meaning getting some kills right after you finish your bloodstone is pretty important. (Note: Be sure to disasemble your arcane boots to get the energy booster for your bloodstone.)
Boots of Travel are a very good choice to turn your boots of speed into after you get your bloodstone as the charges will reduce the time it takes for you to spawn and this will let you get right back into the fight after you do. Scythe of vyse can add some more mana regen and another cc to your arsinal and can be good if your team is lacking that. Shivas Guard will add even more to your plethera of AoE damaging abilities as well as some armor and intellect. Heart of Tarrasque is good if you are in need of some more survivability as the raw strength and hp it provides can be very helpful. Black King Bar i would almost say is a necessity on leshrac as you need to be very close to people to do your damage and if you are being chain stunned that won't happen.
Leshrac is a tier one pick up in ranked play but he is mostly used as a support in very high levels of play.
Leshrac would fit very well into any teamfight or pushing line up. One very important thing about Leshrac as he is part of two of the strongest lanes Leshrac Sand King and the stronger Leshrac Shadow Demon. Sandkings burrow strike can be a very good set up for split earth as well as epicenter combined with Leshracs AoE can be devistating. Shadow Demons disruption and soul catcher combination is almost a constant split earth his aswell as very often a kill.