[[Cauldron of Souls]] is one of the most interesting cards for abusing Undying (and especially in a multiplayer environment) - your creatures can potentially resurrect themselves once using Undying (with a +1/+1 counter), a second time using Cauldron of Souls (with a -1/-1 counter) and a third time with Undying again. It's important to remember that your Undying creatures will be treated as completely new creatures when they re-enter the battlefield, meaning that they won't keep Persist and can only resurrect themselves with Persist once per turn - it's still an extremely powerful interaction. It's a little bit slow for 1v1 games (since getting to 5 mana gives your opponents a lot of time to set their game plan in motion), but it's perfect for slow, grindy multiplayer games where the last man standing wins.

[[Grafted Wargear]] is also an extremely efficient card alongside Undying creatures. Not only does it turn your recurring Undying creatures into powerful beatsticks (Butcher Ghoul becomes a 4/3 that returns as a 5/4, while Hound of Griselbrand becomes a 5/4 Double Striker that returns as a 6/5), but it allows you to repeatedly trigger Flayer of the Hatebound by requipping Grafted Wargear, sacrificing your creatures and reanimating them. Spawning Pit has a similar function - Spawning Pit is useful once you've assembled your infinite reanimation combos (since it can start pumping out tokens), but Grafted Wargear is much more powerful when you haven't assembled those combos and still does a lot of work once you have.

[[Dictate of Erebos]] and the more Black-intensive [[Grave Pact]] are both excellent multiplayer cards - you can pretty much shut down everyone's creatures once Dictate of Erebos is active, which leaves you free to just end the game with Undying creatures. [[Outpost Siege]] is another good option if Dictate of Erebos seems too strong for your group - you either get to 'draw' an additional card every turn (which is extremely powerful in long, grindy multiplayer games) or burn your opponents to death with your Undying creatures, both of which are extremely powerful options in this deck.

As far as general multiplayer cards go, [[Abyssal Gatekeeper]] is a great defensive card, since people will generally avoid triggering it when other opponents are less dangerous targets - even if your opponents kill it immediately, it removes an awful lot of creatures from the board and gives your deck more time to set up combos. It's also fantastic with Cauldron of Souls, since Abyssal Gatekeeper will return with a -1/-1 counter and immediately die, triggering its effect again. While having a few pieces of spot removal around to pick off threats is useful, removal spells that hit multiple opponents is usually more powerful in a multiplayer setting - running a few Tragic Slips alongside cards like Abyssal Gatekeeper, [[Innocent Blood]], [[Barter in Blood]] and potentially Dictate of Erebos should allow you to keep solid control over the board.

Lastly, I'd definitely consider running some more lands - when you're trying to reach 6 mana to cast creatures such as Flayer of the Hatebound and Carnifex Demon, 22 lands isn't likely to be enough. I'd start by going up to 24 lands (which is the baseline for most decks aiming to cast 4-5 mana cards routinely) and consider whether or not running 25 lands would be useful (depending on whether you want Flayer of the Hatebound out early or whether you're happy to wait until later in the game to cast him).