The Hex Dragon is a variety of dragon known as an Arcane Dragon. Basically, Arcane Dragons are dragon species whose abilities focus more towards their inherent sorcerer spellcasting, as opposed to their draconic cousins' more exceptional physical abilities. All Arcane Dragons get Alternate Form from the Wyrmling age category onwards, all of them have a special trait called Graceful Flight where their maneuverability is always at least good, regardless of size (but their movement speeds are about half those of regular dragons), and all of them have a special trait called Physical Frailty where their reach with claw and wing attacks is always lowered by 5 ft if it would be at 10 ft or more due to size, and their natural weapons do damage as if they were a size category smaller.
In exchange, they get faster casting progression than regular dragons, and the two dragons in Dragon Magazine #343 also have ridiculously-increased spell knowledge. ^^; The Tome Dragon automatically knows all Conjuration and Divination spells off the Sorcerer/Wizard list of a level it can cast, while the Hex Dragon has all Necromancy and Enchantment spells.
Other abilities of the Hex Dragon at the age category I'd be getting one at are Deathwatch at-will, Suggestion 3/day, +4 to saves versus death effects, enchantment spells, and spells or effects that deal negative energy damage, immunity to disease and poison, a Retributive Curse ability where it can, once per round, force someone who deals damage to it to make a Will save vs 1d4 rounds of being Sickened or Nauseated, and a breath weapon that's a line of Con-damage Poison.
The main reasons I want the Hex Dragon over the Deep Dragon are the better maneuverability on the flight, the retributive curse ability, and the spellcasting. Also, just for the record, while the extra spell knowledge is powerful, I don't think it will actually be overpowered, given the fact that the dragon will have the casting of only an 8th-level Sorcerer. ^^; The remaining bonus HD from the Paladin levels won't be enough to bump it up by an age category, so it'll always be 4 levels behind the drow spellcasters and 6 behind the half-drow spellcasters.