Dictate of Erebos and Undercity Informer stack question
You must pick another.



The Dictate's ability does not target, so the creature to sacrifice is chosen on resolution.

If you self-sacrifice the Informer in response to Dictate's ability, then it won't exist when the Dictate's ability resolves, so it won't be a valid pick for the Dictate's ability. You'll have to sacrifice a creature among the other ones you have (if any) when the Dictate's ability resolves.

If you wait for the Dictate's ability to resolve before doing anything, the Informer will no longer exist for you to activate its ability.



If the Dictate's ability targeted, the choice of creature to sacrifice would be made when the ability is placed on the stack, meaning you get a chance to respond after the choice is made. If the creature no longer exists when the ability is about to resolve (e.g. because you self-sacrificed it), the ability fizzles and you wouldn't have to sacrifice anything more.