Question: I have a player who is interested in using the Summon Creature boon to summon the undead as their main strategy in a fight. Should I limit where they can use the bane, and if so, how should I go about it? Do they have to summon after someone has already died or wherever they find a corpse? Or should they be able to summon the undead in a field or forest even if there are no obvious bones lying about? Should they be able to summon the undead even when inside a building?
That’s largely up to you as the DM. I’m working on that question myself and the solution I came up with is that if I allow a player summon creatures from the fey wild to magically appear I should let the necromancer summon from some sort of realm of the dead.
I would handle it this way: If a necromancer wants to summon an undead creature he just needs to carry around a sack of bones, that don’t even necessarily match up. If he starts summoning something he throws a few bones out and imbues them with undead energy. Depending on his result on the boon roll, his undead minion will become more and more sophisticated. So, if you only get the lowest CR for the roll you might only get a skeleton arm that drags itself around. If you roll is a bit better you might build the whole upper body, so you have a crawling skeleton, etc. So the roll represents how well the necromancer assembled his temporary minions.
I would only require a full body for the purposes of the animation boon.
But there are certainly other ways to flavor it.
My suggestion is to ask them how it happens when using it. If they come up with a suitably cool and thematic way to do it, its done.
All the previous comments have great suggestions that could certainly be used, and I don’t think you need to restrict the boon specifically due to the undead nature of it.
Let him be able to summon undead in multiple different ways, from the corpses of friends and foes to the souls being ripped into reality from the death realm!