Recently, there has been a few "SSR Units" added into the game.
SSR stands for Super Secret Rare, and there is a 1/10million chance to roll it (rolling 2x 1:10000 in a row to trigger it).
You might not have been so lucky to roll one yourself, but here are some information about the pieces:
SSR Chaos Knight: Costs 15 gold
SSR Necrophos: Costs 10 gold
SSR Enigma (Not released yet)
- These pieces count as the same piece in terms of class/synergy with their regular version pieces (ex: Necro SSR + Necro doesn't stack)
- Unsure of what level you need to be, but seems to appear at 8+