Skeleton King

Ostarion the Skeleton King is a powerful melee strength hero, capable of taking plenty of damage before dying. His ultimate allows him to rise again, acting like an Aegis of the Immortal, making him especially difficult to kill. He also has a nuke that deals reliable damage and stun and he is also able to stay in his lane longer because of his passive lifesteal aura. He can become a hard carry in later stages of game depending on his farm.

Tips

 * Hellfire Blast is a very good nuke, allowing you to disable and slow at the same time. A perfect tool for ganks and team fights, it has a relatively low cooldown, so spam it if you have enough mana.
 * Learning Vampiric Aura during laning phase will shift the creeps equilibrium and you'll be forced to farm and fight under enemy tower. Good for pushing, but hurts your farm and puts you and your allies in disadvantageous position.
 * Always make sure to save some mana for Reincarnation, if you have less than 140 mana, you will not reincarnate. Soul Ring is handy for this purpose.
 * Hellfire Blast is usually maxed first and Reincarnation is taken as soon as possible. It's also recommended to take two levels of Stats on early levels to improve Skeleton King's mana pool and health, as both Mortal Strike and Vampiric Aura are not useful in lane.
 * Armlet of Mordiggian and Soul Ring are commonly built for Skeleton King because Vampiric Aura allows him to heal up quickly after taking damage from these items. With both items, at very low health, you can activate Armlet, then Soul Ring, then quickly deactivate and reactivate Armlet for what is essentially a free Soul Ring use.
 * Activating Phase Boots and Drum of Endurance at the same time will give you enough mobility to chase and kill.
 * Grabbing a Force Staff can help make up for Skeleton King's lack of mobility, and it allows him to jump closer to his targets to make up for the short range of Hellfire Blast.
 * Divine Rapier is a very strong item on Skeleton King, allowing him to take advantage of his great survivability (from Reincarnation and Vampiric Aura).
 * Refresher Orb is situational, as it allows you to potentially Reincarnate twice in a fight allowing you to be that much more useful in a team fight.
 * Although picking up the Aegis of the Immortal may seem redundant on Skeleton King, it is recommended if he is his team's main carry. This way, he has 3 lives to expend, and unlike Reincarnation, the Aegis doesn't require mana.
 * Try to initiate and tank as much damage as possible in team fights. If you are the last to die, nobody will be able to help you when you proc Reincarnation!
 * An alternate build known as wizard skeleton king is actually very viable. Due to the low cool down on Hellfire Blast, by choosing to forgo any points in critical strike for stats, as well as building arcane boots it is possible to have enough mana to constantly spam your stun. Moving from here directly to a dagon makes you a formidable ganker, and allows you to knock out targets in teamfights with less worry of dying compared to a typical nuker due to Reincarnation. Another benefit of this build is that it is very rare to not have enough mana for your ult.

Trivia

 * In DotA, Ostarion was known as King Leoric. Leoric was a recurring boss in the Diablo franchise, another game made by Blizzard. He is an iconic boss in Diablo; he reprises his role in Diablo III, through the alias "Skeleton King."