That would be the Dracolich. First you have dragons. Then you have bigger dragons. These are tougher, older and scarier. Elder dragons are about the most pants-wetting things to appear for any party. I talked about the Godforged Colossus last week, but quite frankly, it hardly inspires the same level of fear as it isn’t as well known. So when you take an elder dragon, and bind its soul to a phylactery, suddenly life becomes much, much more terrifying.
There’s one in Hordes of the Underdark, the excellent expansion to the original Neverwinter Nights, and it is an incredible challenge. Not only is it incredibly tough, immune to all undead immunities, like critical hits and sneak attacks, and in possession of a few spells, this particular dracolich has a special ability. You fight it in a room with several dragon corpses. When you finally bring the massive creature down, the phylactery immediately transfers the soul from the corpse, or something, and the dracolich GETS BACK UP! You have to fight it three or four times!
So yeah, you can keep your adorable little metallic or coloured dragons. I'll go with the one that won't die every time.