Or how about this mechanic - love is the ultimate weapon - the more people you romance, the more power you have when fighting the final boss.
Sounds like a sword of STD, which incidentally should have been the Codex-designed weapon
OkOkOkOkOk - the more people you romance - the more STDs you acquire, the trick is to stay ALIVE while becoming a walking incubator of sexual plagues.
Then you meet and TRICK the final boss to merge with you, ala DX:HR or ME:3 and then they absorb 1,045 STDs from all these various worlds and then they die!
You would be a hero/heroine AND a pariah - as you both destroyed the universe ending final boss AND became the Typhiod Mary of STDs to the whole universe as well.
That would actually be pretty cool.