Soulbound

Soulbound chests are guarded by some enemies before they can be opened. The spectral chains mark those enemies. Often a skeleton key is required to open the chest after defeating the enemies.

"Are you sure about that, just tested on latest version and it didnt not require any and never had required one in past." - Squatch

The loot is expedition and power level depended.

Currently, with Version 0.2.4.0, skeleton keys are better spend on open world skeleton chests.