The Taumet is a magically-created dragon construct, made from five relics whose origins are lost in the mists of time according to the instructions in a book of unknown authorship called the Taumet Codex.
If the Taumet is reduced to zero hit points, it will not be killed, but forced to disassociate into its component relics and the withered remains of the original wearer. The relics cannot be destroyed except by the application of earth-shattering magic (depending upon individual DM's campaigns), but they can be kept separate to prevent the Taumet reforming around another wearer.
Round, AC, HD, Damage, SA/SD
1&2, 5, 1, 1-4/1-4/1-6, Immune to attacks causing less than 4 points of damage
3&4, 3, 3, 1-6/1-6/1-8, Immune to 1st level spells, +1 or better weapons to hit
5&6, 1, 5, 1-6/1-6/2-12, Non-edged weapons cause half damage
7, 0, 7, 1-6/1-6/2-16, Breath weapon usable, +2 or better weapons to hit
8, -1, 9, 1-6/1-6/3-18, Immune to 2nd level spells and attacks causing less than 8 point of damage
9, -2, 11, 1-8/1-8/3-24, +3 or better weapons to hit
10, -3, 13, 1-8/1-8/3-30, Immuned to 3rd level spells
11, -4, 15, 1-10/1-10/3-36, Spell use, immune to attacks causing less than 12 points of damage
12, -5, 17, 1-10/1-10/4-40, Immune to 4th level spells and damage from non-edged weapons
13, -6, 18, 1-12/1-12/5-50, +4 or better weapons to hit
The Taumet can breath 3 times per day, but can choose what form this takes; either as a black, blue, green, red or white dragon.
Once it has the use of spells, it can cast, once per day, those spells that the relics knew at the time of the transformation or 5 x 1st, 5 x 2nd, 5 x 3rd, 4 x 4th, 4 x 5th and 2 x 6th level spells as though it were a 13th level magic user.
Sounds good.Probably the special abilities, as the transformation should be more complex and will probably modify the special abilities to varying degrees.
We'll need to check with Echohawk to find out if there are specific relics associated with the Taumet. If not, we'll need to either create some or offer guidelines to DMs for picking them (as they'll be part of the treasure).
Probably want to bump the DR even more to fit epic 3e specs. Possibly DR 20 and possibly /epic rather than magic.It appears to have at least DR 12/magic and slashing or piercing, some form of immunity to magic, spell use, and a breath weapon.
I'd recommend just modifying the ironwyrm's breath to five varieties.
Ironwyrm has...
Breath Weapon (Su): 60 ft. cone, every 1d4 rounds, 20d10 fire, Reflex DC 30 half. The breath weapon type remains the same regardless of the dragon spirit contained within.
Keep it a cone for all energy types, or match specific shape of the chromatic dragon whose breath it mimics?
Actual spellcasting like many true dragons, or just SLAs?
The RelicsWe'll need to check with Echohawk to find out if there are specific relics associated with the Taumet.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.