Question said:
Thanks. Sadly, you're going to regret posting that...
Right, first things first, I don't see how you're getting 1d6+21 damage. I see +15 (+12 Strength, +3 weapon). Where does the extra +6 come from? (Is that what "Blade Bearer of the Valinar" does?)
Secondly, when wielding the weapon as a double weapon, it is considered equivalent to using a one-handed weapon and a light weapon, meaning that the Strength bonus on the off-hand portion is halved. This changes the damage to 1d6+15/1d6+9. Except...
Double Weapons are made Masterwork and magical one end at a time. Meaning that that weapon doesn't cost 50,425 gold, it costs 100,725 gold. This blows the Wealth-by-Level table out of the water. Did you find that item (in which case it's a DM mistake), or did you buy it? In the latter case, you really need to make some alterations.
Thirdly, your attack bonuses with the weapon are incorrect. However, I suspect you won't mind this one: they should be +27/+27/+22/+22/+17 (+25 base, -2 for two-weapon fighting, +3 weapon, +1 weapon focus).
Finally, there's are two items worn in the 'wrong' body slot there: Bracers of Natural Armour and Vest of Resistance. That's fine, but doing so costs extra (I forget the mark-up), precisely to prevent the casual stacking of items that aren't meant to be stacked (Amulet of Nat Armour and Amulet of Health).
Basically, that character is not rules-legal, so it's not surprising that the DM has run into problems. I don't know the history of the character, so can't say where the problems have come from (the DM may have given this stuff out without thinking, or you might have presented that character, and he approved it). Either way, for the good of the game, I suggest you go through the character sheet with a fine-toothed comb, and correct all the problems.
I suspect that the DM's problem isn't really with the Belt of Strength, but rather with the character as a whole. Correcting the rules errors for that character should therefore fix the problem.
If anything it is the template that is overpowered, although it doesnt really have any bearing on the damage(half-dragon also gives +8 racial str)
Not having Races of the Dragon, I can't comment. However, assuming it's LA +2 or higher, it doesn't seem too bad.
The DM is(judging from his old posts) highly experienced, running multiple games, and is supposedly a member of Dungeon(Or dragons, not sure) staff. Or used to be.
A lot of DMs claim a lot more capability than they actually have. Given the number of inconsistencies I spotted so quickly, I would be take his claims with a pinch of salt.