Arch's guide to chapter construction goes over the basic idea of how conditionals work in GBAFE, but it's focused toward FE7.
The conditional codes for FE8 are in the documentation for EA (which you should read because it will help you learn it on your own).
How conditions work in FE8 can be found on my eventing resource thread.
You'd probably have to run a turn event that's checking if they're alive or not every turn, and if they're not it does whatever you're trying to get it to do.