my initial impression is that it is insanely powerful. we buy this item with the INTENTION of deleting a health pool.
incidentally, if you do not remove the debuff within 0.36 seconds, a well timed cursed relic with superior duration will ensure you lose most of your health pool. you can't use items when cursed if i recall
heres the issue with equippable hard counters: they tear up metagames. players stop thinking about which soft counter to buy and just go for the hard counter, meaning you'll farm for the same items every match. everyone needs to get fingers, then dispels, and divine barriers. any hero who currently gets soft countered by dispel magic suddenly gets a huge buff because theres a new debuff in town that demands your cooldown.
this is a pretty hard countering item because the damage increases automatically as people gain hp and the cooldown is shorter than dispel magic
this item is extremely beneficial for grouped up teams that can force doom to activate. so actually, it will grant extreme amounts of power to graves/rem/mirage who can shove lanes and join teammates faster than anyone else. a snowballing team will now have a very easy way of continuing to snowball walkers down unless the defending team interrupts their build to also acquire the same item. then we lose item choices.
the item doesn't just force people to abandon their dive, it forces them to abandon their current position outright. so it can be exceptionally good at ruining any defenders advantage. I wonder how this affects midboss..
but thats just talking about EQUIPPABLE hard counters. because the boring part is that EVERYONE can and will buy it.
I think you've designed a perfectly good ult concept for a future hero though
