Now unsure if it will work (since it may be too inconsistent against solid decks) so any suggestions to make it more solid is welcome. At least concept's out there.

Fiendish Servant is the only Deathrattle minion in your deck, there should be no other Deathrattle minions. Have it die.

Then use Malchezaar's Imp with Tome Tampering to shuffle your combo into your deck, and keep drawing as much of the combo as you can, there are a few control spells with an Imp package to keep an early lead and allow a power spike with Vile Library in case you gather your combo early. Augmented Elekk is a backup in case you want to have multiple chances.

But its as follows, Imp King Rafaam + Baron Rivendare + Tamsin's Phylactery/Argent Horserider (can be in any order) and then Soul Rend.

Horserider is the only one that should live, and have the Deathrattles go to him. Soul Rend ensures your minions are destroyed but Horserider is also alive, you could swap this with a Hellfire for a generalist use against other matchups, but make sure Rafaam is not infused as your imps will have more than 3 Health in most cases.
In this case you should have a few pings with you just in case it rolls on the wrong minion, but Phylactery should always have Fiendish Servant, so you can also pull this off with Lady Darkvein, for a double deathrattle, albeit its a little harder to pull off.