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.