Fun fact! The blood thorn is one of the few sessile D&D monsters with a dexterity score!
Not-so-fun fact: the blood thorn is one of the few sessile D&D monsters.
3 plant HD, Large size, surprisingly good physical stats (ranging from +6 to +10), reach, four 1d8 tendril attacks, improved grab that activates after two tendril hits (and deals constitution damage when ended), and blood drain.
On the one hand, that's a nice package of physical stats. On the other hand, you're a plant without hands, most item slots, intelligence, and movement (also, arguably, sight or blindsight). Getting this to move is tricky enough already (several Talismans of the Disk?), let alone having it contribute meaningfully to noncombat encounters (or, depending on your attitude towards grappling, encounters of any kind).
While the blood thorn with some modifications might be worth using, it's definitely not viable as it is right now. -0 LA.