Darkness is a core design feature for Warlocks, but for a player to effectively leverage their class advantage both the player and DM must know, understand and implement the RAW surrounding darkness properly. What follows is a summary of the relevant RAW and a brief discussion of the implications of that RAW as relates to Warlocks in particular.
Darkness Spell:
Magical darkness spreads from a point you choose within range to fill a 15-foot radius Sphere for the Duration. The darkness spreads around corners. A creature with Darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. If the point you choose is on an object you are holding or one that isn’t being worn or carried, the darkness emanates from the object and moves with it.
Devil’s Sight:
You can see normally in Darkness, both magical and nonmagical, to a distance of 120 feet.
PHB pg 183 “Vision and Light”:
“A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition (see appendix A) when trying to see something in that area.”
Blinded Condition:
– A blinded creature can’t SEE and automatically fails any ability check that requires sight.
– Attack rolls against the creature have advantage, and the creature’s Attack rolls have disadvantage.
Unseen Attackers and Targets:
– When you attack a target that you can’t SEE, you have disadvantage on the attack roll.
– When a creature can’t SEE you, you have advantage on attack rolls against it.
Opportunity Attacks:
You can make an opportunity attack when a hostile creature that you can SEE moves out of your reach.
Ranged Attacks in Close Combat:
When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can SEE you and who isn’t Incapacitated.
PHB p. 177 for Passive Perception penalty in dim light. PHB 183 for disadvantage on active Perception checks when in dim light
So to summarize….
A warlock with Devil Sight could cast darkness on himself, and then just walk up to people and:
-gain advantage on all attacks since no one can see him, and if he has rogue levels, also triggering sneak attack once per TURN.
-all attacks against him would have disadvantage, and if hidden would have to guess his actual location or fail automatically.
-can hide anywhere at any time, even while moving in the open.
-immunity to attacks of opportunity.
-may make range attacks in close combat without suffering disadvantage.