The thing is, it's sometimes REALLY hard to hurt or kill PCs for some DMs.
Not for me. Each time I play this campaign PCs have died (just out of luck) including with this group. I do have 'back in points' I add when this happens and let them roll a new PC (at a lower level than the rest of the party) and introduce them back in the story. For this group, one of them separated themselves from the rest of the group in the battle, went down to 0HP and then their first death save roll was a "1" - it was fun watching the others trying to get to him to save him (only to fail when he next rolled a 4).
So, a question. Mechanically, in what way is he invincible. Has he literally got "Immunity, all damage"? If so, even if the players go up to fight him, you will describe their hits as appearing to just pass through him, leaving him unharmed. As for the heart, well, I wouldn't make it as simple as you intend...
As I say, its pretty much like HP and the horcrux. The mechanics is that with the heart still beating, she (its a she by the way) regenerates hit points pretty fast as a bonus action and has immunity to non magical attacks. there is one magical user who could possibly do enough damage, but the way he uses his spells, he will be pretty out of decent spell slots for the big battle. Now I could lower the regen dice down, but again, its a cheat so I don't want to do it.
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