[Twan]

You can make a new one but it's rather complicated.

First/easier method is to replace an existing spell (if you mod a nation replacing a national spell of a nation of another era shouldn't make problem).

A more complicated method allow to use any new spell.

You need to use 2 mods, one with the new spell, one calling it for onebattlespell. Use for example the spell enabler mod (posted here somewhere by Endorperez iirc) which should be your first mod enabled.

Make your self kill effect and insert it in the spell enabler mod.

Call this spell by number in your other (second) mod.

Note that this will only work as long no new spell are added in vanilla game (numbers will change when spells are added).