I don't think there is an established collective noun for this fictional creature. Unicorns have somewhat an established collective noun because unicorn is a well-known legendary/fictional animal. Blessing can fall under terms of venery and these terms are more prevalent when we are more familiar with the animal and if they are used more frequently in literature.
Having said that, one can use their imagination to coin a collective noun for liches and it would be more suitable if fiction writers do it. You can also use different collective nouns depending on the context.
For example, One of the writers used scores:
The demon waddled into the main hall on its short legs. There were scores of liches.
Lord of the Isles, Volume 1 By David Drake
And one used chorus:
"The Weave... collapsing," answered the chorus of liches.
The Ghost King: Transitions, Book 3 By R.A. Salvatore
But in the end, you can always use group itself like some writers do.
I could also find some collective nouns used in fantasy role-playing game related texts.
From Tibia Wiki:
Woe and behold! A plague of Liches threatens Drefia's Deeper Vampire Crypts.
From Baldur's Gate Wiki:
Through armies of skeletons, legions of zombies, hordes of noncorporeal undead, and a gauntlet of liches they battled.
I could also find a gaming forum discussion where people are trying to find collective nouns for legendary/fantastic creatures and there are two suggestions for liches:
- a pall of liches
- a lynch of liches