Can warlock undead form a pact with Falazure? or Shar?

My character was exiled in to the Shadowfell and she want to return to Material Plane but i don't know which greater deity should I patron with.

After i search for my goal in google and I found one named "Falazure" but he isn't in the Shadowfell, Instead he is in Plane of Carceri

And then I seatch for more, I found "Shar" or "Mistress of the Night" but she also not in the Shadowfell, Instead she live in Astral Sea

So i have a question, Can I form a pact with one of them?
If not, is there any name should I form a contract with?

Ask your DM. Who and what your warlock patron can be is really up to them.

Technically, both of those are gods, and as a rule, gods don’t have warlocks, they have clerics. But, as wagnarokkr says, ask your DM.