I feel like if my deck has Demons I don't really want the first ability to make huge creatures, because Demon's are huge creatures. If you just remove the BB ability, though, I think it becomes an interesting design (a variant on the Ring of Evos Isle cycle).
It does sell "book of dark magic that makes you powerful at a cost commensurate to the power you wean" pretty well. I agree that just the first ability could be fun on its own; I wonder whether the complete package would probably want to be named "Xarlehcioh's Tome" a la Scroll of Griselbrand, to highlight This Guy Is Plot-Important.
I think the Ring cycle makes pretty clear that one +1/+1 counter per turn is no where near enough to make something potent. Mayhaps instead, we could have:At the beginning of your upkeep, reveal the top card of your library. Put X +1/+1 counters on equipped creature and you lose X life, where X is that cards CMC.
The ring cycle was strong in Limited, as long as your colors matched up.
Could Dark Confidence as an aura cost B? Better just as an ability on a creature?
Definitely inferior to the pure design, but players love callbacks to old broken cards.
*The ring cycle was tolerable in limited, as long as your deck was heavily slanted. Go listen to LRs from that period, they were very low on the Rings. [NB: The Rings were at uncommon, and had they been costed to be good, it would have been miserable, because playing against "huge unstoppable creature you can't kill" isn't very fun, so I think their power level was (correctly) set very low. For a single thematic rare, though, the power level could have been set much higher. ]
Thinking you should be able to sacrifice any cultist OR a creature with sufficient power, so the counters mean more to the Tome.
You could also have something along the lines of "exile cards until you reveal a demon with power less than equipped creature's power, then put it into play." That way the counters mean something to the cultists, too.Of course, then the tome no longer cares about cultists as a tribe. Maybe make it so the first ability is the one that only works on cultists?
a different design coming from a place of "corrupting, powerful", plus punishing you for using it in the wrong way:Equipped creature gets +4/+0At the beginning of your end step, equipped creature deals 4 damage to you.
or this is probably better wording for getting the message acrossEquipped creature gets +4/+0At the beginning of your end step, if ~ is equipped to a creature, you lose 4 life.
The abilities seem pretty at odds with one another. Why does the same artifact that asks me to attach it to a creature for slow +1/+1 counter gains also let me sacrifice my creatures w/o caring about their power at all?
Black magic.And because it lets you kill your monster before it kills you.