The most deadly megabeast I've modded is this:
Body:
[BODY:SIX_SPLITEND_TENTACLES]
[BP:RUT:right upper tentacle][CON:UB][liMB]
[BP:LUT:left upper tentacle][CON:UB][liMB]
[BP:RUTC:right upper claws][CON:RUT][GRASP]
[BP:LUTC:left upper claws][CON:LUT][GRASP]
[BP:RMT:right middle tentacle][CON:UB][liMB]
[BP:LMT:left middle tentacle][CON:UB][liMB]
[BP:RMTC:right middle claws][CON:RMT][GRASP]
[BP:LMTC:left middle claws][CON:LMT][GRASP]
[BP:RLT:right lower tentacle][CON:UB][liMB]
[BP:LLT:left lower tentacle][CON:UB][liMB]
[BP:RLTC:right lower claws][CON:RLT][GRASP]
[BP:LLTC:left lower claws][CON:LLT][GRASP]
Creature:
[CREATURE:GREATER_DEMON]
[NAME:Greater demon:Greater demons:demonic]
[TILE:'&'][COLOR:4:0:1]
[MODVALUE:50]
[FANCIFUL][ENDING][POWER][DEFENDER]
[INTELLIGENT][FIREBREATH][DRAGONFIREBREATH][FIREIMMUNE][FIREIMMUNE_SUPER][liKES_FIGHTING][MAGMA_VISION][trANCES]
[AMPHIBIOUS][UNDERSWIM][SAVAGE][IMMOLATE]
[FLIER]
[trAPAVOID]
[MEGABEAST]
[WEBBER][WEBIMMUNE][THICKWEB][AMBUSHPREDATOR][PARALYZEIMMUNE][STOUT]
[GENPOWER:5]
[FREQUENCY:5]
[EXTRACT:Greater Demon Tail Venom:20:0:1]
[EXTRACT_PARALYZE]
[EXTRACT_PERMANENT]
[EXTRACT_STUN]
[EXTRAVISION]
[BONECARN][PET_EXOTIC]
[BLOODTYPE:O]
[EVIL]
[DAMBLOCK:10]
[BUILDINGDESTROYER:2]
[NOPAIN][EXTRAVISION][NOFEAR][NOEXERT][NOSTUN][NONAUSEA][NOBREATHE][NOTHOUGHT][NOSKULL][NOBLEED][NOSKIN][NOBONES]
[NOMEAT]
[CANOPENDOORS][LOCKPICKER]
[PREFSTRING:beauty]
[PREFSTRING:infernal magic]
[PREFSTRING:corrupt intentions]
[BODY:HUMANOID:2WINGS:SIX_SPLAYEND_TENTACLES:TAIL:TAIL_STINGER:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THR
OAT:NECK:SPINE:BRAIN:2HEAD_HORN:5FINGERS:5TOES:MOUTH]
[SIZE:20]
[ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:4:18:BURN][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:LHORN:gore:gores:2:8:GORE][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:RHORN:gore:gores:2:8:GORE][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:RUTC:stab:stabs:2:3:PIERCE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:LUTC:stab:stabs:2:3:PIERCE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:RMTC:stab:stabs:2:3:PIERCE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:LMTC:stab:stabs:2:3:PIERCE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:RLTC:stab:stabs:2:3:PIERCE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:LLTC:stab:stabs:2:3:PIERCE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:25:50]
[ATTACK:MAIN:BYTOKEN:RUT:slap:slaps:2:5:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:LUT:slap:slaps:2:5:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:RMT:slap:slaps:2:5:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:LMT:slap:slaps:2:5:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:RLT:slap:slaps:2:5:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:LLT:slap:slaps:2:5:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTYPE:GRASP:claw:claws:2:4:SLASH][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTYPE:STANCE:kick:kicks:2:4:BLUDGEON][ATTACKFLAG_WITH]
[ATTACK:MAIN:BYTOKEN:TAIL_STINGER:sting:stings:5:20:PIERCE][ATTACKFLAG_CANLATCH]
[SPECIALATTACK_INJECT_EXTRACT:50:100]
[NO_SLEEP][NO_EAT][NO_DRINK]
[EQUIPS]
[FAT:3]
[ALL_ACTIVE]
[SPEED:650]
[STANDARD_FLESH]
[HOMEOTHERM:10095]
[SWIMS_LEARNED][SWIM_SPEED:8000]
[PERSONALITY:ANXIETY:0:0:0]
[PERSONALITY:DEPRESSION:0:0:0]
[PERSONALITY:SELF_CONSCIOUSNESS:0:0:0]
[PERSONALITY:VULNERABILITY:0:0:0]
[PERSONALITY:FRIENDLINESS:0:0:0]
[PERSONALITY:ASSERTIVENESS:100:100:100]
[PERSONALITY:CHEERFULNESS:0:0:0]
[PERSONALITY:TRUST:0:0:0]
[PERSONALITY:STRAIGHTFORWARDNESS:0:0:0]
[PERSONALITY:ALTRUISM:0:0:0]
[PERSONALITY:COOPERATION:0:0:0]
[PERSONALITY:SYMPATHY:0:0:0]
[SPHERE:FIRE]
[SPHERE:DEATH]
[SPHERE:TORTURE]
[CURIOUSBEAST_EATER]
[CURIOUSBEAST_GUZZLER]
[CURIOUSBEAST_ITEM]
[HEATDAM_POINT:50000]
[IGNITE_POINT:50000]
[LEATHER_HEATDAM_POINT:50000]
[LEATHER_IGNITE_POINT:50000]
[BONE_HEATDAM_POINT:50000]
[COLDDAM_POINT:0]
Entity:
[ENTITY:GDEMON]
[CREATURE:GREATER_DEMON]
[ADVENTURE_TIER:7]
[INDIV_CONTROLLABLE]
[ABUSE_BODIES]
[ETHIC:KILL_ENTITY_MEMBER:PERSONAL_MATTER]
[ETHIC:KILL_NEUTRAL:ACCEPTABLE]
[ETHIC:KILL_ENEMY:ACCEPTABLE]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:ACCEPTABLE]
[ETHIC:TORTURE_FOR_INFORMATION:ACCEPTABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:ACCEPTABLE]
[ETHIC:TREASON:PUNISH_CAPITAL]
[ETHIC:OATH_BREAKING:PERSONAL_MATTER]
[ETHIC:LYING:PERSONAL_MATTER]
[ETHIC:VANDALISM:PERSONAL_MATTER]
[ETHIC:TRESPASSING:PERSONAL_MATTER]
[ETHIC:THEFT:PERSONAL_MATTER]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:PERSONAL_MATTER]
[ETHIC:EAT_SAPIENT_OTHER:PERSONAL_MATTER]
[ETHIC:EAT_SAPIENT_KILL:PERSONAL_MATTER]
[ETHIC:MAKE_TROPHY_SAME_RACE:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:ACCEPTABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:ACCEPTABLE]
Thankfully they only rarely appear and rule over goblin fortresses, but damn they are seriously dangerous, the webbing, firebreath and blood drinking abilities are only in there because at present there isn't any magic, I also included some code so they can be played in adventure mode, so far I've only managed to kill one once with my most powerful adventurer but she was mortally wounded and died soon after.