An item that boosts your monsters the fewer tier 1+ synergies you have would be a great addition. Adding +5 speed may be a bit too overpowered, as (for example) if you had that item when you had seven synergies that weren't tier 1+, each of your monsters would gain +35 speed which is ridiculously overpowered in addition to the +70 atk, def, and hp. Probably a +3 or +2 speed instead would be better, and also gaining int too. Overall, this is a great concept, and has the chance to be really overpowered, especially with the reforging and merging possibilities with the forge. Nerfing this even more should probably happen, as this has the possibility of becoming really powerful really quickly, especially if someone manages to find two in a single run. I like the unique idea of an item that instead of buffing synergies by adding one to a monster, it buffs monsters based on synergies a player doesn't have.
Hmmm, I'm not sure how useful an event specifically for one item could be, if there's nothing else to be gained from it. Personally, I feel the word "Sacrifical" would entail losing max HP, which is already a feature found in other events. Presumably, it would be a random monster losing a percentage of their max HP, say around 10%. Something that would make it more interesting is to tie it in with the snake dealer somehow, given that their theme is also about sacrificing something to gain something else. Even better, have it be a random chance of getting similarly themed items like the vampire fang or that blood axe (that makes the monster lose 10 HP every time it attacks), so that it's not a guarantee of getting a legendary item 100% of the time.
The other option of losing one synergy (probably not a tier) instead of max health is also very unique, and I would like that feature to show up (unless it's random, in which case I would be more hesitant to accept). The thing is, there's no point in really getting it from the event if you can also get it from battles (or shops), and if it's the same rarity as all the other legendary items. With all the events that already exist, it might be more likely to get it just from other places besides that singular event. Also, the only other event that can guarantee a legendary item (to my knowledge) is the one where you have to fight a battle as well, so the drawbacks would have to be quite substantial for this new suggested event.
Also, I personally think that should add 5 int each time too. There are some items that only give str and def without int, and actually many of them don't have int, but given the point of this item, it should probably also give int just to round things out.
I mean, it was you who came up with the idea in the first place, it probably just needs some changing to make it less overpowered but personally I think it's quite cool.
It's also not my game, so it's really Jay's executive decision on whether to add something similar to this into the game.
I'm just a person active in this game's comment section attempting to balance this item so it can more easily be integrated into the game's based on its already existing mechanics and themes, I've got no control over whether the item does get added and I don't really want to come up with a similar concept already made by someone else.
Perhaps we should seek Jay's input on this item (with the suggested changes).
*Hint Hint*
:)