I've tested this specifically in the test map and even if you get 1 stack of djinn's mark over the threshold it still won't trigger tankbuster or mystic burst. So it seems to be a deeper bug with Djinn's mark not procing this item. (or is intentionally exempted for some reason)
I wasn't thinking the Soul sacrifice upgrades would work per stack. So a flat 30% increase +50 per box. The 7+ 'per stack' upgrades would only apply to the normal stats (hp, weapon damage, spirit power etc)
Also (if it wasn't clear) the active has a soul cost that doubles per use so 7 stacks...
This item is meant to explore some of the other design space around items that generate soul income that have now been introduced. ('Trophy Collector' and 'Cultist Sacrifice')
Overview: A cheap spirit item with an Active that consumes an (increasing) amount of the players souls each use in...