Weapon enchantment that increases damage die?

[atemu1234]

I remember reading something about one somewhere, I just can't remember where.

[Khatoblepas]

Alchemical Gold/Platinum, the special material from Magic of Faerun? That's all I can think of.

[Ratatoskir]

The spell (Greater) Mighty Wallop permanencied? Or just cast 1/day for the greater version would probably work at higher levels

[Techwarrior]

The sizing enhancement sort of does that, by actually making your weapon bigger.

[Uncle Pine]

If you really want to pump your weapon's damage die you could also combine enlarge weapon (CS) and sizing, in this order: enlarge weapon enlarges your weapon by one size and enables you to wield it regardless of its size. Add sizing for Colossal greatswords on Tiny characters (1 round/level though, so you might want to persist it or to get an unlimited use magic item of it).