There is a relic axe, Cronos, which does 4d2 + 14 damage. (It also adds 100 hit points and takes 50 off your luck, neither of which would have much effect on a high-level character.) Apart from that, the best axe would be a Supreme Axe (4d2 + 11 damage, +11 to hit), which is considerably inferior to either Poseidon or the Elite Halberd (3d6 + 10, +10 to hit).You can find weapons with the enchantment "of the Dragon", (+(10 to 20) points of fire damage and +25 might), to increase the axe's potency, but you can do exactly the same to the halberd so that makes no difference.
