Sir Jemima, the Living Shade

Sir Jemima takes the form of a smoky crystal in the
cross-guard of your sword, which becomes a +1
weapon. While bearing the weapon you always have
two shadows, one of which looks like a slender mouse
in armour.
Twice per long rest, you can choose to have the
crystal absorb half the HP damage from any attack.
If you hit another creature with the sword before the
end of your next turn, you can add that damage, as
Shadow Damage, to your own weapon damage.