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Every 50th kill of this enemy drops Goblin Warlock Banner.they come out of the warlocks
Nothing spawns these naturally. A Goblin Warlock throws them out in batches during a goblin invasion and keeps throwing for as long as it lives, so the swarm is the symptom and the caster is the problem. Clearing them does nothing for your invasion counter either, which means every second spent on the swarm is a second the rest of the army spends walking toward your base unopposed.
why cover makes it worse
They ignore terrain completely and fly straight at you through whatever you are hiding behind, and knockback does not slow them at all. Both they and the warlocks move fast enough to close a gap before you have finished reacting. Boxing yourself in costs you your own firing angles while doing nothing about what is incoming. Keep sight lines open, drop each warlock the moment it shows up, and no more apparitions get made.