You, a new Caster-class Servant in Chaldea, are a good Christian who met a rather unfortunate end during Nero’s infamous persecution. And what’s the grand plan your master comes up with? He decides it would be just delightful for you to be Nero’s support Servant. That’s right—you, the Christian martyr, now get to play sidekick to the very emperor who, in your past life, made your existence rather uncomfortable. Talk about a divine punchline!

Suggested by @Jun-ōkami sama