Level 3
Spirit Reliquary.
As a reaction, when a creature dies in combat, you may offer its Soul to your Patron; keep note of them (Solo Monsters count as Legendary Souls).
Animanipulation.
When summoning a shadow, you may use Pilfered Power to imbue it with all traits and abilities, excluding HP, from a stored regular Soul up to your LVL; different Souls must be commanded separately. The Soul is lost if the shadow dies.
Level 7
Spectral Syphon.
As an action, you can consume any amount of stored Souls. To your next Shadow Blast’s, add +1 damage for each regular Soul and 1d12 for every Legendary Soul (ignoring armor). Whenever any of your shadows would take damage, you may take it instead.
Level 11
Amalgamated Idol.
(1/Safe Rest) 2 actions: Fuse all your shadows into one, then imbue it with a Legendary Soul up to your LVL (it requires 2 actions to command). It survives a number of hits equal to the shadows fused: it is Bloodied at half its hits (rounded up), and gains 1 extra hit upon Last Stand.
Level 15
Siege of the Fallen.
(1/encounter) 3 actions: Deal 6d12 damage + KEY (ignoring armor) to ALL enemies in a Cone 2. Plus 1 Cone size for every 10 stored Souls (Legendary Souls count as 5).