If by "serve death for all eternity" you mean be like an assistant, like fetch his clothes from the dry cleaners and getting him coffee everyday, I don't think I'd go for that. Being Death's errand ***** for all eternity would not be fun.
But I'm guessing Death doesn't need those things, so it might be something cool like taking over for him once he gets out of the business of soul harvesting and retires to Europe or something.
If so, I'd definitely want him to make me ruler of the world for as long as I live. Which is guaranteed to be at least 10 years, I can get some stuff done.
Once that happens, I'd abolish wars, feed the hungry, redirect some funds to space travel and make new laws like instituting a time of the day set aside for naps, and maybe one for hugs as well.
World peace and happiness for everyone, and I get to be the Damn Immortal God-King of Earth and all the Stars, everybody wins!