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.
Once the process of creating the Taumet has begun, it takes 13 melee rounds to develop into the complete Taumet. During this time, the wearer of the relics is used as the basis of the Taumet's body, dying in the process.
The Taumet takes some of the wearer's memories and alignment during its growth, always remaining evil, but taking the Lawful, Neutral or Chaotic attitude of its 'parent'.
Like all normal dragons, the Taumet has a set number of hit points per hit dice, but this depends on the original wearer of the relics:
Wearer; hp/HD
MU or Illusionist; 4hp
Thief, assassin; 6hp
Cleric, monk, druid; 8hp
Fighter, ranger, paladin; 10hp
Let's ditch it then. As an alternate, we could replace the usual construct bonus hp with the "parent's" hp.
During the transformation period, how do we treat the HD line? I don't think a 10HD barbarian, for example, should drop to 1HD in the first round of the transformation.