Silences the enemy for 3 sec. If used on a casting target, it will counter the enemy's spellcast, preventing any spell from that school of magic from being cast for X sec.
Spell Lock is a Warlock Felhunter skill, best described as a hybrid of Silence and Counterspell. When used, it will silence its target for 3 seconds; if the target was in the process of casting a spell, however, then the spell will be interrupted and, depending on Spell Lock's rank, the target will be unable to cast a spell from the same school of magic for either 5 or 6 seconds. This is an instant-cast ability with a 24-second cooldown.
Tips and Tactics
Spell Lock, like all of the Minion skills, can be set to Auto-cast; however, as the AI will normally use this skill to immediately Silence its target, rather than attempt an arguably more beneficial interrupt, it might be more viable to disable this. Disabling auto-cast will allow the Warlock to decide if a spell is worth interrupting, or if the target is enough of a threat to use an interrupt on.
Use of this skill will usually force an NPC caster to enter melee combat, where they are at a considerable disadvantage.
Patch 2.1 - "...duration reduced to 5 seconds (rank 1) and 6 seconds (rank 2), and cooldown reduced to 24 seconds." (Previously it was a 6/8 second duration with a 30 second cooldown.)