If you’re Horde side, definitely start an association with Undercity Nexus. You will not be disappointed! As an Alliance DK, I have even had enjoyable encounters with it’s members.
https://us.forums.blizzard.com/en/wow/t/h-rp-undercity-nexus-forsaken-rp-guild/942897
They also host an “Undead Social” most Thursdays, and ALL Undead type RP Characters are welcomed!
https://us.forums.blizzard.com/en/wow/t/hn-rp-undead-tavern-night-every-thursday/955382
Also, for something more DK specific, there is the Discord of the Damned. Admittedly I have been a bit scarce in lately, so I can not speak to their current activities.
https://us.forums.blizzard.com/en/wow/t/discord-of-the-damned-a-dk-rp-community/1056338
Hope these help!