This thing.
[CREATURE:YA_TE_VEO]
[DESCRIPTION:A large monster from the evil forests, resembling a tree with many long tendrils for branches. Beware its vile cursed sap!]
[NAME:devil tree:devil trees:devil tree]
[CASTE_NAME:devil tree:devil trees:devil tree]
[BIOME:TAIGA]
[BIOME:ANY_FOREST]
[CREATURE_TILE:198][COLOR:5:0:0]
[POPULATION_NUMBER:15:20]
[CLUSTER_NUMBER:1:10][LOOSE_CLUSTERS]
[EVIL]
[LARGE_PREDATOR]
[GRASSTRAMPLE:0]
[NOT_BUTCHERABLE]
[NOT_LIVING]
[IMMOBILE]
[NOSMELLYROT]
[NOPAIN][EXTRAVISION][NOBREATHE][NOSTUN][NOFEAR][NONAUSEA][NOEMOTION][NOTHOUGHT][NOEXERT]
[NO_DRINK][NO_EAT][NO_SLEEP]
[NO_DIZZINESS][NO_FEVERS]
[BODY:BASIC_1PARTBODY:5ROOTS_STANCE:TWELVE_NO_CLAW_TENTACLES]
[NO_THOUGHT_CENTER_FOR_MOVEMENT]
[USE_MATERIAL_TEMPLATE:WOOD:WOOD_TEMPLATE]
[STATE_NAME:ALL_SOLID:devilwood]
[STATE_NAME_ADJ:ALL_SOLID:devilwood]
[STATE_COLOR:ALL:PURPLE]
[DISPLAY_COLOR:5:0:0]
[SOLID_DENSITY:1500]
[MATERIAL_VALUE:5]
[PREFIX:NONE]
[TISSUE:WOOD]
[TISSUE_NAME:wood:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:WOOD]
[MUSCULAR]
[FUNCTIONAL]
[STRUCTURAL]
[RELATIVE_THICKNESS:1]
[VASCULAR:5]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[TISSUE_LAYER:BY_CATEGORY:ALL:WOOD]
[USE_MATERIAL_TEMPLATE:SAP:PLANT_EXTRACT_TEMPLATE]
[STATE_NAME:ALL_SOLID:frozen devil tree sap]
[STATE_NAME_ADJ:ALL_SOLID:frozen devil tree sap]
[STATE_NAME:LIQUID:devil tree sap]
[STATE_NAME_ADJ:LIQUID:devil tree sap]
[STATE_NAME:GAS:boiling devil tree sap]
[STATE_NAME_ADJ:GAS:boiling devil tree sap]
[DISPLAY_COLOR:4:0:0]
[PREFIX:NONE]
[ENTERS_BLOOD]
[SYNDROME]
[SYN_NAME:devil tree curse]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_IMMUNE_CREATURE:YA_TE_VEO:ALL]
[SYN_CONTACT][SYN_INJECTED][SYN_INGESTED]
[CE_PAIN:SEV:75:PROB:100:RESISTABLE:START:50:PEAK:1000:END:2000]
[CE_NAUSEA:SEV:75:PROB:100:RESISTABLE:START:150:PEAK:1000:END:2000]
[CE_VOMIT_BLOOD:SEV:75:PROB:100:RESISTABLE:START:250:PEAK:1000:END:2000]
[CE_BODY_TRANSFORMATION:START:2000]
[CE:CREATURE:YA_TE_VEO:DEFAULT]
[CE_ADD_TAG:NO_AGING:OPPOSED_TO_LIFE:START:2000]
[ATTACK:SLAP:BODYPART:BY_CATEGORY:TENTACLE]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:slap:slaps]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_WITH]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[BODY_SIZE:0:0:200000]
[BODY_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[BLOOD:LOCAL_CREATURE_MAT:SAP:LIQUID]
[CANOPENDOORS]
[ALL_ACTIVE]
[SPEED:1120]
[SWIMS_INNATE][SWIM_SPEED:2500]
[ITEMCORPSE:WOOD:NO_SUBTYPE:LOCAL_CREATURE_MAT:WOOD]
[CASTE:DEFAULT]
Long story short, it's an immobile evil living tree. When you kill it, you'll probably get covered in its sap, which at first gives you intense pain, nausea and chronic coughing, and then
forcibly turns you into another evil tree. While you've long gone inside fortress walls.
Also, sentient corpses from evil forests can turn into these too.
[INTERACTION:EVIL_TRANSFORMATION_FOREST]
[I_SOURCE:REGION]
[IS_REGION:FOREST]
[IS_REGION:EVIL_ONLY]
[IS_REGION:SAVAGE_ALLOWED]
[IS_FREQUENCY:100]
[I_TARGET:A:CORPSE]
[IT_LOCATION:CONTEXT_REGION]
[IT_AFFECTED_CREATURE:DWARF:ALL]
[IT_AFFECTED_CREATURE:ELF_WOOD:ALL]
[IT_AFFECTED_CREATURE:ELF_LIGHT:ALL]
[IT_AFFECTED_CREATURE:HUMAN:ALL]
[IT_AFFECTED_CREATURE:DUNEKIN:ALL]
[IT_AFFECTED_CREATURE:USIRIM:ALL]
[IT_AFFECTED_CREATURE:GOBLIN:ALL]
[IT_AFFECTED_CREATURE:ORC:ALL]
[IT_AFFECTED_CREATURE:LIZARDMAN:ALL]
[IT_AFFECTED_CREATURE:CENTAUR:ALL]
[I_EFFECT:RESURRECT]
[IE_TARGET:A]
[IE_INTERMITTENT:WEEKLY]
[SYNDROME]
[CE_BODY_TRANSFORMATION:START:0]
[CE:CREATURE:YA_TE_VEO:LARGE]
[CE_ADD_TAG:NO_AGING:OPPOSED_TO_LIFE:STERILE:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
EDIT: fixed one of the the CE_ADD_TAGs