Unlike other spells that get some boost when its rarity improves, Soul Siphon has a static 10% per skull gem effect. Please consider changing the spell to have a rarity-related effect.
Thanks!
Hey @Zudaio
Soul Siphon is designed to have a static 10% per Skull Gem. I will pass that feedback on to the Development Team 
OminousGMan - Support Human 
That’s fine but then there is no part of the spell influenced by rarity, unlike every other spell in the game…
From the shadows was the second. No part is affected by rarity.