What happens if you use Death Worship and Betrayers for a Kingdom?

I do not know, but you could start a game with two AIs, one of which is headed by a death mage (for example, Ceresa) and force that one to surrender.

What is likely to happen: The Shard changes to death and you have to raze the shrine / outpost and rebuild it at which point it becomes a life shard. Had odd stuff happen with razing AI outposts with death-shrines being built in their area influence when putting down an Outpost / Monolith the same turn . But technically it should allow you to go massively into life-shards (maybe to avoid gamey workarounds it should be checked if a kingdom or an empire has the trait and a purification spell given instead for kingdoms, possibly even a whole set of life-oriented spells for the trait alternatively make kingdoms unable to take the trait).

I've discovered what happens - it changes to an unbuilt death shard, and stays an unbuilt death shard until you reload, when it becomes a life shard.  But Kingdom champions (including sov if a Kingdom) can't appear to upgrade Death Magic on levelup (6 or 7 levels of testing, so not conclusive, but likely) but if you hire a Death Magic hero, that gets Death Magic 2 from levelups or starts with at least Death Magic 2, you can do it.  (So obviously you need to have picked Betrayers.)

Even though it doesn't affect the base game , for the sake of modding I'd like to see the devs implement better rules for betrayers and life/death shards.

On eof the reasons you may not see heroes with death magic is:

#1 it is a rare trait, probbaly maybe three heroes tops with it, just like life magic.

#2 the Ai hired them.

#3 the opossing faction encountered and killed them. The AI will kill unhired heroes fo the opposing faction.

The second map I tried had heroes with Death Magic, thanks.