Note: based on Table 48 of the 2E AD&D DMG. Should be used as a guideline but can be adjusted for balance purposes. Specifically, the attacks of clearly magical creatures (i.e. mephits) should at least count as +1 weapons.

Creature Hit Dice  Weapon Enchantment

4+1                     +1
6+2                     +2
8+3                     +3
10+4                    +4
