Okay, here's a first draft of the Strangler. It's based on a downsized mindless Tendriculos with some of the tentacle text of a giant octopus.
I've added fire vulnerability as suggested by Shran.
The description's "It is unaffected by either darkness or bright light, and doesn't react to sound or silence ... it will never attack a motionless creature that it hasn't already captured with at least one tentacle." suggests it is blind, deaf and have tremorsense, which I've added in.
I've arbitrarily reduced the tentacle damage to 1d4 from the original's 1d8, since that fits the tentacle/tendril damage of a Large Giant Octopus or a Tendriculos reduced to Large. We can always reinstate them, but I'd prefer the 1d8 damage tentacles to be reserved for the Gargantuan version.
Strangler
Large Plant
Hit Dice: 6d8+24 (55 hp)
Initiative: +1
Speed: 0 ft. (immobile)
Armor Class: 16 (-1 size, -1 Dex, +6 natural) touch 10, flat-footed 16
Base Attack/Grapple: +4/+13
Attack: Tentacle +8 melee (1d4+5)
Full Attack: 8 tentacles +8 melee (1d4+5)
Space/Reach: 10 ft./10 ft. (20 ft. with tentacles)
Special Attacks: Constrict 1d8+7, improved grab, paralysis, swallow whole
Special Qualities: Blind and deaf, camouflage, plant traits, tremorsense 40 ft., vulnerability to fire
Saves: Fort +9, Ref +3, Will +1
Abilities: Str 20, Dex 13, Con 18, Int --, Wis 8, Cha 3
Skills: ---
Feats: ---
Environment: Any plains?
Organization: Solitary
Challenge Rating: 4?
Treasure: None
Alignment: Always neutral
Advancement: 6-7 (Large), 8-12 HD (Huge); 13-18 HD (Gargantuan)
Level Adjustment: —
Combat
Stranglers attack with their tentacles, which they use to crush opponents before attempting to swallow them. A strangler can attack a single target with up to 6 tentacles.
Opponents can attack a strangler's tentacles with a sunder attempt as if they were weapons. A strangler's tendrils are AC 13 and have 10 hit points each, but damage inflicted to a tentacle doesn't count against the strangler's normal hit point pool. If a strangler is currently grappling a target with the tentacle that is being attacked, it usually uses another tentacle to make its attack of opportunity against the opponent making the sunder attempt. The creature regrows severed tentacles in 1d6 days.
Blind and Deaf (Ex): A strangler can neither see nor hear, so is not affected by attacks based on vision or hearing, such as a medusa's petrifying gaze.
Constrict (Ex): A strangler can automatically deals 1d8+7 points of damage with a successful grapple check.
Improved Grab (Ex): To use this ability, a strangler must hit an opponent of any size with a tentacle attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can constrict and swallow whole.
Paralysis (Ex): Any creature that a strangler constricts, swallows or hits with a tentacle must make a DC 17 Fortitude save or be paralyzed for X rounds. The save DC is Constitution-based.
Swallow Whole/Paralysis (Ex): A strangler can try to swallow a grabbed opponent of a smaller size than itself by making a successful grapple check. Once inside, the opponent takes 1d10+5 points of crushing damage per round plus 4 points of acid damage, as well as being exposed to its paralysis attack. A swallowed creature can cut its way out by dealing 20 points of damage to the plant's interior (AC 13). Once the creature exits, muscular action close the hole; another swallowed opponent must cut its own way out. A Large strangler's interior can hold 2 Medium, 8 Small, 32 Tiny or 128 Diminutive smaller creatures.