I think that, ideally, the DV should offer some bonuses for various tiers of paladin levels, paladin PrEs and the capstone, and should be upgradeable to become the best-in-slot 1-hander for a pure paladin.
As far as upgrading goes, I'd imagine that it would require, say, 5 Large Devil Scales, 9 Trophies of War, 10 Epic Raid Tokens and something else...something on-theme and requiring players to tackle tough, high-level content. Maybe a Seal of the Abbot? A Martial Cannith Cell? A Refined Spirit of whatever? I'd rather not generate new items that drop in stuff like Elite Tower just to upgrade this thing. Perhaps, add a named item to the Epic Chrono end chest that is useful for everyone, and required in the upgrade ritual for the sword?
As for the effects, I'm thinking:
Base sword, is a normal +5 cold iron longsword
Paladin 6 adds
- Holy
- Axiomatic
- SR 20
- Smite Evil recharge reduction of 5 seconds
Paladin 12 adds
- Holy Burst
- Axiomatic Burst
- SR 25
- Smite Evil recharge reduction of 10 seconds
Paladin 18 adds
- SR 30
- base damage increase to 2d8
- Smite Evil recharge reduction of 15 seconds
Capstone adds
- SR 35
- x3 crit multiplier
- Smite Evil recharge reduction of 20 seconds
- Radiance
- +1 to paladin auras (AC, saves, Concentration, Fear)
- Greater Dispelling (only dispels buffs; does not remove our debuffs/DoTs)
Epic Upgrade adds
- +6 enhancement bonus
- 3d8 base damage
- SR 40
- Smite Evil recharge reduction of 25 seconds
- Evil Bane (+2 enhancement, +2d6 vs. evil creatures)
- Chaos Bane (+2 enhancement, +2d6 vs. chaotic creatures)
- Radiant Blast (as on Epic Wraps of Endless Light)
- Red slot
Defender of Siberys adds
- Tier I - 5% incite; count as 2 levels higher for LoH damage
- Tier II - 10% incite; count as 4 levels higher for LoH damage
- Tier III - 15% incite; count as 6 levels higher for LoH damage; Recharge 1 Lay On Hands per 2 minutes
Knight of the Chalice adds
- Tier I - Smite Evil Recharge reduction by 5 seconds; count as 2 levels higher for smite damage
- Tier II - Smite Evil Recharge reduction by 10 seconds; count as 4 levels higher for smite damage
- Tier III - Smite Evil Recharge reduction by 15 seconds; count as 6 levels higher for smite damage; Outsiders struck by smite must make Will save or be banished, DC = 30+Cha (creatures immune to banishment take extra 150 damage)
Hunter of the Dead adds
- Tier I - +5% Healing Amp; recharge Remove Disease 1/minute
- Tier II - +10% Healing Amp; recharge Turn Undead 1/minute
- Tier III - +15% Healing Amp; Undead struck by smite must make Will save or be destroyed, DC = 30 + Cha (creatures immune to disruption take extra 150 damage), weapon gains Greater Undead Bane