Your formless Patron exists outside of mortal understanding.
My attempt at a "default" Shadowmancer Subclass, since their didn't really seem to a true Necrotic-focused Shadowmancer Subclass either in the Heroes book or on the Nexus.
Level 3
A Knowledge Forbidden.
Your Patron grants you even MORE forbidden knowledge. When you choose a Lesser/Greater Invocation, you may also choose another Invocation of the opposite type. Your shadow minions become faceless humanoids made of darkness.
Lightless Protectorate.
Shadow Minions within Reach 4 may Interpose for you.
Level 7
Gloomclaw.
(2xDEX/Safe Rest) When using Shadow Blast, you may instead form a dark claw and attack all creatures within Reach 1. Invocations affecting Shadow Blast also affect Gloomclaw.
Persistent Penumbra.
Your Shadow Blast instead does 7+KEY damage (ignoring armor), +7 every 5 levels.
Level 11
From the Murk.
At the beginning of your turn, summon 1 adjacent Shadow Minion for free.
Level 15
Master, I Beg of You!.
(1/Encounter) Whenever you use Pilfered Power, you may immediately cast another tiered spell equal to half your currently known tier of spell (round down) for free.