First of all, Rrath, Rilla and Nor stop right now

With that out of the way, the party, through some shenanigans has found themselves the darklords of a domain of dread. I am kinda freaking out because darklords can’t leave the Shadowfell through normal means and I kinda need them to be on the material plane. Would fey shenanigans work I need suggestions on how, without breaking the multiverse, the characters could return to the material plane. HELP. 

YA know what planar residents hate?

Being summoned.

Personally, I'd say they can't. I wouldn't tell them that, of course. I'd run the campaign as if they were on the prime material plane, dropping subtle hints and omens, and at the very end reveal in some dramatic and horrifying manner that they're still in ...Shadowfell? The Domains of Dread, at any rate.

I think in the lore vecna escaped the shadowfell? maybe look it up and see if you can find anything to help there?

In my campaign, I've tweeked a few things, so in CoS Strahd wants Ireena but in less of a creepy old man way. He want to use her assuming she's bound to him being stuck in Barovia so hopes to enact a ritual to escape. I thought that since the ravenfolk and vistani can travel the mists, along with heroes who bimble around that over the centuries Strahd has discovered a vast amount of information on planar travel (mostly through torture). I am also playing with the idea that dark lords can't return to the material plane but maybe the fey or elemental planes? and from there to the material plane.

In my campaign strahd is harvesting souls that would usually return to the mist and be reborn to fuel the ritual as when released there won't be enough soulless bodies or new born to host these souls and the resulting explosion of released energy will tear a hole into another plane. This is helped by the fact my players are all exotic races so a few could by deemed items from other planes used to anchor the ritual

Domains of dread are kind of like a personal hell.Firstly creature can't leave because it requires external assistance whether it be a god or some one outside doing a ritual. Secondly they are usually also distracted by some personal failure that the domain uses to torment them. So, some one has to let them out and they have to over come some kind of personal failing.

Though arguably players should not really be bale to become dark lords unless they are just the absolute worst people possible and overcoming  personal failure is impossible for them.

LOL we switched DMs after we became darklords guess the previous DM didn’t look into the lore that much.  *shrugs* I’ll figure it out. Thanks to you all for your advice.