Fallen Angle can be bulit for free

When I chose a unique card,I Found Dark Lucifer's price is 0. So I chose it and built it in the 1st turn. It kills normal waves quickly but hardly to kill a boss. So I do not know it is a bug or a feature...

https://mazebert.com/forum/bugs/fallen-lucifer-choosable-by-skill--id1251/ This is already a known but.