MANA
Natural energy in all living things. Everyone has it, few can control it. Mainly used by sorcerers and mages, but anyone talented enough can learn it for daily use.
WILL
A warrior's strength and experience combined. Unlike mana, it's not guaranteed. It stays latent until forced out by great suffering or great achievement.
DEMON MAGIC
Demons are the only race that naturally uses mana and will together. Other races can learn it, but it doesn't come as easy.
DEMON CURSES
When a demon dies, it can curse its killer, binding a unique curse/sigil to their soul.
Sigils can stack.
Sigils can be altered (not removed) by someone skilled enough in both mana and will.