Once again it's important to realize that poison as a damage keyword is quite unique: It actually has non-damaging effects under its purview. Necrotic does not. Even if your power is necrotic and poison, it's irrelevant because the necrotic doesn't count for the non-damaging parts of the power whatsoever. The poison part WILL take over those non-damaging parts of the power - so an immune creature now ignores them.
Noting that the power will gain necrotic + poison, which may matter in the case of a poison immune (but not necrotic anything) creature. It won't be able to resist the poison damage anymore, but the non-damaging effects are still ignored.
Noting that the power will gain necrotic + poison, which may matter in the case of a poison immune (but not necrotic anything) creature. It won't be able to resist the poison damage anymore, but the non-damaging effects are still ignored.