☀️ Arselia and Akonia

We are all God's slaves...

Does it necessarily have to be "legal" slavery? Or could it be mental slavery, for example? Could it apply not to the player, but directly to a princess, or another NPC?

According to the rules, you can't own slaves here, so mental slavery is implied – at the level of being unable to disobey or harm (loving your master, for example, is not necessarily required). And yes, it can apply to a princess or another NPC.