Forge Spurned monster advancement

[Gwaednerth]

Okay, so one of the locales in my current world is an ancient, cursed dwarven fortress in the far north, called Motte Moraine. The short version is that it's full of evil, twisted, eternally cursed dwarves. Most of these are going to be duergars, possibly with a couple class levels here or there, and undead templates. But for the boss, I want to use a Forge Spurned (from Crown of the Kobold King). However, the Forge Spurned is only CR 5. I can give it some class levels, maybe throw on a template, but the really interesting thing to do would be to advance its soul chain.

For those of you unfamiliar with the monster, it has a chain that it forges out of the souls of its victims. The chain serves as armour and as a weapon, and gains +2 armour bonus and +1 weapon enhancement bonus per 10 links (10 souls). Now, since nobody who ventures to Motte Moraine ever returns, I thought it would be cool to give the Forge Spurned a gigantic chain, far longer than the measly 30 links of the statblock in CotKK. But how many links is equal to +1 CR? Should a chain that immense have additional powers, such as constricting, entangling, etc.?

[MesiDoomstalker]

Well, determine first the CR you want him to be at. Then compare the AC and attack routines of creatures of the same CR. Add the appropriate amount of links to match that CR benchmark. But your critter will have poor literally everything but AC and maybe to-hit bonuses. Adding Classes or HD will help make up for it to some degree. Adding some kind of Constrict and/or auto-Grapple would be appropriate for a massive chain.