Mnemon

Thetis had warned Achilles that if ever he killed a son of Apollo, he must himself die by Apollo's hand; and a servant named Mnemon accompanied him for the sole purpose of reminding him of this. But Achilles, when he saw Tenes hurling a huge rock from a cliff at the Greek ships, swam ashore, and thoughtlessly thrust him through the heart … Achilles put Mnemon to death because he had failed to remind him of Thetis's words …
Apollo sought out Paris in the thick of battle, turned his bow and guided the fatal shaft. It struck the one vulnerable part of Achilles' body, the right heel, and he died in agony. But some say that Apollo, assuming the likeness of Paris, himself shot Achilles.
— Robert Graves
Achilles, that violent brat,
had leapt from the boat,
swum ashore, thrust a spear-shaft
through Tenes. He's like that.
Now it's Mnemon's turn
for not reminding him
never to kill any son
of Apollo …
An unhauntable man,
Achilles gives Mnemon not
another thought: the servant,
Memory, lying derelict,
unhonoured.
To the one who can be murdered,
but never stopped, I offer
this epitaph: Who carries your
memory, carries your fate.
Achilles' heel, Apollo's spear —
checkmate.