Sounds like exceptional reach, improved grab, constrict, and acid. Seem about right to you?
I'd vote against Constrict, since they seem to do damage purely by exuding acid on creatures its holding.
There's also no mention of them having a particularly high strength, so we may want to give them a racial bonus to grapple, or maybe Improved Grapple as a bonus feat?
Note the 2nd edition stats' Special Attack line describes it as "entanglement", so maybe we should use that as a model instead of constriction?
Anyhow, statswise I'm thinking it looks something like:
Magen, Caldron
Medium Construct
Hit Dice: 4d10+20 (42 hp)
Initiative: +2
Speed: 30 ft. (6 squares)
Armor Class: 15 (+2 Dex, +3 natural), touch 12, flat-footed 13
Base Attack/Grapple: +3/+5
Attack: Slam +5 melee (1d2+2 plus 1d4 acid) or
weapon +5 melee (
X+2)
Full Attack: 2 slams +5 melee (1d2+2 plus 1d4 acid); or slam +6 melee (1d2+2 plus 1d4 acid) and
weapon +5 melee (
X+1)
Space/Reach: 5 ft./20 ft.
Special Attacks: Acid,
entangle [or constrict], improved grab
Special Qualities: Construct traits, damage reduction 10/bludgeoning, darkvision 60 ft.,
resistance to acid?, low-light vision, spell resistance
X
Saves: Fort +1, Ref +3, Will +1
Abilities: Str 14, Dex 15, Con —, Int 9, Wis 11, Cha 10
Skills: Listen +3, Spot +4
Feats: 2
Environment: Any
Organization: Solitary, [
elasticity? solution?] (2-8), or concoction (2-12 magen of different types)
Challenge Rating: 2
Treasure: No coins; double goods (gems only); double items
Alignment: Always neutral
Advancement: 5-6 HD (Medium)
Level Adjustment: —