Frankly,
Do it with skill checks and keep it out of combat, there is no combat mechanic for controlling an army because 4e is dependent heavily on the action economy that would break hard by giving a PC multiple undead allies with their own actions.
Basically, the tactical combat play in 4e is reliant on balance, and what your proposing will break the balance, hard.
However, let's talk about the skill check option here. What your opening up is the potential for a couple of sessions of largely narrative play. It will primarily involve skill checks and challenges and is a huge opportunity for your players to embrace the role playing aspects that 4e does actually do quite well.
However, for this, you're going to have to throw the combat aspects of the rule book out the window and go with your imagination and story telling aspects. There are explicit not rules for this (intentionally so), instead there is the skill system, which you can use to support the narrative play that is called for here. So develop some skill challenges that fit and have a good idea of a couple of different plot threads that you could take and then see where your players and rolls take you.
If you want the actual mechanical guts of this: the sword provides the location of the ritual (make it up) and a short skill challenge with religion checks (easy DCs) should get him a decent army depending on his checks, after that, what he does with it should be a series of narrative cut scenes, your end goal should be to advance the character, but if you want to continue the game as a traditional 4e exercise you've got to get him back to just being him.
They integrate as well as any other hybrid1. There are no rules forbidding the combination, and the fact of published hybrids of essentials and dragon-only class hybrids (both for the original and essentials executioner assassin) existing suggests this same conclusion.
The literature supports this conclusion, with the hybrid handbook making no distinction between "essentials/nonessentials" save for the actual hybrid options themselves.
Fey pact warlock and executioner is actually a published build called the Eladrin Eldritch Executioner which provides:
- Play a melee character who emphazises wits over brawn
- Be stealthy and have at-will invisibility/hiding from early paragon onwards
- Be a capable addition to the party for skill challenges of a social, information gathering, or infiltration variety.
- Have very solid single target DPR at all teirs of play (reaching about 150 at-will DPR, by 30).
- Get to make a lot of OAs by using stealth offensively (at least 1 most rounds, unless fighting solos) and be a Warlord's best friend.
Nothing here looks particularly objectionable, as the hybrid seems to be a way of gaming stealth, rather than flopping over more than one role.
1 Mostly really horribly and worth ignoring, but with a few decent combinations.
Best Answer
Simple: he is a true believer of the true Pelor, The Burning Hate.
It’s just fanon, but it’s a pretty fun one. Might make for a really cool character, and a very interesting plot hook. Depends whether or not you had important plans that hinged on Pelor being as described in the books.