I figured out an exploit to make Horcrux users even more invincible.

A. If you make a Horcrux, you cannot be destroyed unless your Horcrux is destroyed.

B. People can be Horcruxes.

QED if one person turns another into a Horcrux and the other reciprocates then they have foolproof immortality.

This method also has three other benefits over the Dark Lord's attempt, that I can think of. First, it requires only two murders, not seven. Second, it causes twice as many people to become immortal. Third, you'll retain a much larger portion of your soul than you would otherwise, and avoid much of the consequent degeneration.

Quirrelmort is playing on the level below mine, clearly.