This question asks about the maiming weapon enchantment, which was updated in Magic Item Compendium: Miniatures Handbook: If the weapon normally has a ×2 critical multiplier, roll 1d4 each time you successfully score a critical hit to determine your multiplier. For weapons with a ×3 multipli...

As noted in David Coffron’s answer to another question, the moonblade (Dungeon Master’s Guide, pg. 217) has no fixed limit on how much damage it can do: there is no stated limit to the number of runes on the blade. It states: A moonblade has one rune on its blade for each master it has served...