Large construct (shapechanger)

HD: 7d10+45
Initiative: 0
Speed: 30
AC: 17 (-1 size, +8 natural), touch 16, flat-footed 16
Base Attack/Grapple: +6/+22
Attacks: Slam 2d6, Constrict 3d8
Full Attack: 2 Slams 2d6
Space/Reach: 10/0
Special Attacks: Improved Grab, Constrict, Suffocate
Special Qualities: Darkvision 80 ft, Construct traits,
Magic Immunity
Saves: Fort +1, Ref +1, Will +0
Abilities: Str 26, Dex 22, Con 13, Int 15, Wis 13, Cha 17
Skills: Climb +24, Jump +19, Move Silently +20,
Spot +15, Swim +23, Tumble +20
Feats: Alertness, Combat Reflexes, Dodge, Mobility
Environment: Underground
Organization: Solitary
Challenge Rating: 8
Treause: None
Alignment: Always chaotic
Advancement: As a fighter
Level Adjustment: --

An Alarus can draw up to (or drop onto) a foe and then wrap around him, immediately squeezing the victim for 3d8 damage.

Alaruses can also choose to suffocate the victim for 1d10 nonlethal damage/round if wrapped around the neck, or the entire victim.

The Alarus is a thin sheet of metal, about 20 ft by 15 ft. It is hardly reflective at all,
but can take on any Large hollow shape it wishes or even become a solid
Medium character, such as a half-elf fighter. An Alarus usually attacks by transforming
into a mundane, everyday item, such as a rope - and wrapping around and suffocating the
creature once near.

Alaruses will attack anything on sight, regardless of alignment or size.

So what do you think? First time ever creating a monster. It was originally going to behave like a swarm, but then it'd be nigh-invincible, and the only way I can think of to defeat it is to capture it in a Bag of Holding or something...

Oh, and I'm not sure about the CR, could someone please help me with that?