Wraiths are flying enemies introduced in GemCraft Lost Chapter: Frostborn Wrath. They first appear on field X4. They resemble apparitions but are made of flesh.
They don't attack and don't leave the field, so they can be killed whenever for their bonus drops, such as Shadow Cores.
It is however recommended to kill them as soon as possible as each wraith reduces the damage taken by all monsters by 15% per wraith and can stack up to 90% if there are multiple wraiths on the field.
The appearance of a wraith is preceded by a dark flashing effect at the start of the few preceding waves that includes two vertical lines in the middle of the screen.