Never underestimate age and experience, Keepers. Especially when they're clothed in the robes of a Warlock, encountered in an enemy's dungeon, hurling fireballs in your direction!
A ranged combatant that is able to lob fireballs over Walls and other obstacles, Warlocks themselves have fairly low HP, so they're a bit "glass cannon", in concept. Possessing average movement speed, Warlocks are somewhat susceptible to Poison damage... but given their overall low HP, it doesn't take much to lay them low!
(As we've discussed in previous articles, fire damage is particularly devastating to Trolls, so keep that in mind!)
To begin recruiting Warlocks to your cause, you simply need to build a Dark Library (see our recent feature article, just prior to this one!). Upgrading Warlocks improves their HP and damage, and brings about some visual changes, as well.
Strategically, Warlocks are really good at creating forward deploy points safely hidden behind a wall, and/or being able to destroy rooms without damaging other Minions.