Sword of Xylytyx

An artefact belonging to the Elderday race of Xylytyx, "the Sword" is one of the few known remnants of an ancient civilisation that controlled the third and fourth iterations of Atlantis. The Sword may be attuned by a creature of 25th level or higher and takes up three attunment slots:

is a standard sword
"until" the controller uses "movement" or an "action" to activate it (taking force damage equal to controller proficiency bonus x5", which cannot be redirected or prevented)
stays active for a number of rounds equal to the proficiency bonus x5 in rounds
deals 2d10 damage of a damage type determined by the wielder at the top of each round (before the highest initiative acts)
may add a level of exhaustion (as a bonus action) to increase range of attack to proficiency bonus x 10 for one round
may take 20% damage of maximum hit points (as a reaction) of wielder to deal maximum damage for one round with each succesful attack
gains +2 attack rolls
gains +6 attack rolls for one round, by giving advantage to all attack rolls against the wielder for three rounds (cannot use this ability again until the end of three rounds)
Each occassion the wielder activates the Sword of Xylytyx their hit point maximum is reduced by their proficiency bonus x2 after their next long rest