Warlock

Although the Warlock is a powerful support hero, his offensive capabilities should not be underestimated. With a strong physical attack and his Shadow Word ability, the Warlock is tough to beat early game. Shadow Word can be used either offensively to deal damage over time, or defensively to heal an ally. Thanks to this versatility, the Warlock can often be found supporting allies against difficult enemies, using his Shadow Word to heal his lane partner or harass an enemy hero. Later in the game the Warlock's other spells come into play. Fatal Bonds links enemy units together, causing them all to take extra damage when one is attacked. This spell is especially effective at increasing the potency of allied area of effect spells. Although somewhat difficult to use well, Upheaval can be a powerful skill in certain situations. If given time to channel, Upheaval is a potent slow with a large area of effect. The Warlock's primary strength, however, is summoning a powerful Infernal with his devastating Rain of Chaos spell. When cast, this ability drops a meteor on the target area of effect, stunning and damaging in a very large area, as well as summoning the powerful demonic Infernal. This ability makes the Warlock great for initiating battles. After being stunned and attacked by a massive Infernal, his foes will likely have little will left to fight.

Source

Basic Strategy

 * Warlock is one of the best babysitters in DotA. Shadow Word allows him to heal his lane parner and Upheaval slows down enemies giving your partner a chance to escape or gank. Try to lane with carry heroes such as Spectre, Phantom Assassin to boost their early-game.
 * Consider going solo lane if your team wants to try early push strategy. Warlock's attack range, projectile speed, and damage allows him to farm nicely against other solo heroes.
 * Shadow Word can also be used as a harrassing tool. Cast it on your opponent and keep hitting him whenever it's possible. Make sure you don't get hit by creeps too much while doing so.
 * Fatal Bonds is an immensely usefull spell which amplifies your team's damage output. Bond 6 enemies together, and if they are all hit by a single AoE spell or attack, the total damage will be doubled, since everyone of them will suffer 5x20% extra damage echoed from others.
 * Initiate team fights by casting Rain of Chaos.
 * Use your golem summoned by Rain of Chaos to push towers, clear creep waves, attack weaker enemies during teamfights.
 * You can heal the golem with your Shadow Word to increase his tankiness.
 * Do not attack fleeing enemies with your golem, it will slow you down. Just keep the golem running close to them and let the immolation do the work for you.

Trivia

 * Warlock is voiced by Sam A. Mowry.