Bad Player
Deity
Currently good AI civs will give you a negative diplo modifier if you have death or entropy mana. However this doesn't happen with human players and anyway it seems a little arbitrary.
My idea:
Each entropy mana source in the world causes degradation of the world (e.g. a low % chance each turn of a random improvement or building being destroyed).
Each death mana source in the world similarly has a low % chance each turn of causing death (e.g. killing a random living non-hero unit).
This would give non-death/entropy civs a reason to hate those who use death/entropy magic. People would use death/entropy anyway if the benefits to their civs outweigh the damage it does to themselves. This fits in with the Ashen Veil theme of damaging all other evil/neutral civs more than themselves and therefore is still beneficial even though it does some damage to themselves.
My idea:
Each entropy mana source in the world causes degradation of the world (e.g. a low % chance each turn of a random improvement or building being destroyed).
Each death mana source in the world similarly has a low % chance each turn of causing death (e.g. killing a random living non-hero unit).
This would give non-death/entropy civs a reason to hate those who use death/entropy magic. People would use death/entropy anyway if the benefits to their civs outweigh the damage it does to themselves. This fits in with the Ashen Veil theme of damaging all other evil/neutral civs more than themselves and therefore is still beneficial even though it does some damage to themselves.