Warlock

Demnok Lannik the Warlock is a ranged intelligence hero and a good support. His Shadow Word ability makes for an excellent heal as well as a decent harassment tool, and his physical attack helps him to ward off any enemies and keep his friends safe. He also possesses a considerable presence in team fights, as most of his abilities cover up a rather large area of effect.

Tips

 * Consider going solo lane if your team wants to try an 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 harassing 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 useful spell which amplifies your team's damage output. Bond 5 enemies together, and if they are all hit by a single area of effect spell or attack, the total damage will be doubled, since everyone of them will suffer 5x20% extra damage echoed from others.
 * You can use Upheaval to prevent wounded enemies from escaping. This is especially useful if used right after an area-of-effect spell, such as your Chaotic Offering. Let your golem or your teammates finish the job.
 * If an ally is being chased, use your Upheaval to slow his chasers. Since the slow will last for some seconds after you stop channeling, you will also have time to fall back.
 * When initiating, make sure to cast Fatal Bonds first, then Chaotic Offering; this way, you can output more damage against all your targets.
 * Your golem is tanky; when possible, use it to tank creeps or towers, so that you can dispose of them sooner.
 * You can heal the golem with your Shadow Word to increase its tankiness.
 * When chasing fleeing heroes with your golem, don't stop to auto-attack them, since it will slow the golem down. Instead, keep it moving with the heroes, so that the Permanent Immolation can do its damage.
 * Although both Refresher Orb and Aghanim's Scepter allow you to summon two golems, Refresher Orb allows you to summon two full strength golems and utilize two seconds of AoE stun, but will cost a lot of mana to pull off, while Aghanim's Scepter is easier and cheaper to build and gives nice stats.
 * Do not let the golem die needlessly to your enemy heroes, since its gold bounty is quite high. If it's the last remaining unit for your team in a fight, have it flee unless you can ensure a kill. Having it die against creeps or towers is better than allowing an enemy hero to claim the gold bounty of your golem. Having two golems from Aghanim's Scepter will cut the bounty of each golem in half.
 * Fatal Bonds and Venomancer's Poison Nova make for a deadly combination, as although Poison Nova cannot kill on its own, Fatal Bonds will cause anyone bound and affected by the poison to die from the damage ticking off.
 * Remember your role: as a support Hero, you should be leaving as much experience and gold as possible to the carry. While items like Aghanim's Scepter and Refresher Orb may seem so powerful, it's advised not to buy them too early, or not to buy them at all: concentrate on buying Wards, Dust and utility items like Mekansm, Force Staff, Necronomicon, Drum of Endurance or Eul's Scepter of Divinity first. Even Wards and Mekansm alone will certainly be more useful than a Warlock who stole kills from the carry, leaving him underfarmed and underleveled. Only acquire kills if the carry is unable to do so.

Trivia

 * In DotA, Warlock's ultimate is named Rain of Chaos and the summoned unit was called an Infernal, a hulking golem with green-colored flames. Both names were changed in the transition to Dota 2 to avoid copyright conflict.
 * Warlock is related to Disruptor and  Axe, as he comes from the Oglodi tribe.