According to 3rd Edition rules, an ordinary Holy Avenger has the following properties:
- Holy descriptor (add 2d6 bonus damage to evil creatures; this damage is considered holy)
- Spell resistance 15, 5-foot radius (incoming spell must roll higher than 15 on 1d20 + caster's level + caster's spell-driving ability modifier or the spell fails)
- Cast dispel magic, 5-foot radius, once per round as standard action (does not target any spell in particular)
- +5 bonus to attack and damage
- The sword is by definition made of cold iron, which allows it to more easily penetrate the defenses of demons (also fey, though the Chaotic Evil demons are clearly the primary intended target).
For any nonpaladin, this works only as a plain +2 longsword.
The 3rd Edition Epic Level Handbook introduces an even more absurdly powerful weapon, the Holy Devastator. In the hands of nonpaladins, this works as a +3 holy longsword (yes, it keeps the holy descriptor). Give it to a paladin, though, and watch the fur fly:
- Increase bonus to +7 (this isn't even allowed by standard game rules)
- Replace holy descriptor with Holy Power descriptor (+3d6 bonus holy damage and inflict one negative level on evil creatures; both these are affected by critical hits)
- User gets +5 bonus on all saves against spells with Evil descriptor, or all spells cast by evil characters)
- A paladin using the smite evil ability with this weapon adds double his/her paladin level to the damage (normally adds just paladin levels).
The market price is over 4.5 million GP for a reason, folks. To be fair, this same handbook also offered the Unholy Despoiler, a version of the Holy Devastator for blackguards (anti-paladins). However, there is nothning analogous to the Holy Avenger for these characters yet.
Thanks to Jay Digital for correcting me: Holy weapons deal 2d6 extra holy damage to evil creatures, not 1d6.