Boss Spawners are an objects that you can find them in Titanium Chests or craft them at the Crucible using:

Boss Spawner - Yandeon (10 Spirit Sticks, 10 Ice Shards)

Boss Spawner - Shindeon (5 Red Fire Sticks, 10 Dark Shards.

They can summon a Boss that is around Level 400-600.

If you want to sell them, the max price you can get for them is:

Boss Spawner - Yandeon (170.000+. To sell this you need Huge Inventory from Mutant Market (15 gems)).

Boss Spawner - Shindeon (120.000+).