My last attempt at contributing failed, but with this back I'll try to write SCP-939 now that I can into creature modding (kinda.) It seems like it'd be possible to implement with a fair degree of accuracy, and it presents a reasonable danger.
Edit: Here's SCP-966, because I couldn't figure out how to do 939 properly. I don't know how common they should be, so I've left things like population and clustersizes up to you.
The creature itself:
[CREATURE:SCP-966]
[DESCRIPTION:An anomalous humanoid creature. It is invisible to the naked eye and weakens its prey by depriving them of sleep.]
[NAME:SCP-966:multiple instances of SCP-966:SCP-966]
[CREATURE_TILE:'&'][COLOR:3:0:0]
[CANOPENDOORS]
[AMBUSHPREDATOR]
[NOFEAR]
[NOSLEEP]
[NO_VEGETATION_PERTURB]
[NOT_BUTCHERABLE]
[PREFSTRING:invisibility]
[PREFSTRING:patience]
[BODY:HUMANOID_NECK:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:ORGANS:HUMANOID_JOINTS:THROAT:NECK:SPINE:BRAIN:SKULL:5FINGERS:5TOES:MOUTH:TONGUE:FACIAL_FEATURES:TEETH:RIBCAGE]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[USE_MATERIAL_TEMPLATE:NAIL:NAIL_TEMPLATE]
[USE_TISSUE_TEMPLATE:NAIL:NAIL_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:FINGER:NAIL:FRONT]
[TISSUE_LAYER:BY_CATEGORY:TOE:NAIL:FRONT]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SKIN:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:STANDARD_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RIBCAGE_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RELSIZES]
[USE_MATERIAL_TEMPLATE:SINEW:SINEW_TEMPLATE]
[TENDONS:LOCAL_CREATURE_MAT:SINEW:200]
[LIGAMENTS:LOCAL_CREATURE_MAT:SINEW:200]
[HAS_NERVES]
[USE_MATERIAL_TEMPLATE:BLOOD:BLOOD_TEMPLATE]
[BLOOD:LOCAL_CREATURE_MAT:BLOOD:LIQUID]
[CREATURE_CLASS:GENERAL_POISON]
[GETS_WOUND_INFECTIONS]
[GETS_INFECTIONS_FROM_ROT]
[USE_MATERIAL_TEMPLATE:PUS:PUS_TEMPLATE]
[PUS:LOCAL_CREATURE_MAT:PUS:LIQUID]
[CAN_DO_INTERACTION:SCP-966]
[ADV_NAME:sleep deprivation]
[CDI:USAGE_HINT:ATTACK]
[CDI:USAGE_HINT:TORMENT]
[CDI:VERB:glare menacingly:glares menacingly:NA]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:5] (half the range given in the article, for balance reasons)
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:108000] (can use this once per season. This is because their syndrome is usually fatal, and it would be silly to allow one of these things to kill everyone at the site before you even know what's going on.}
[BODY_SIZE:0:0:4000]
[BODY_SIZE:1:168:15000]
[BODY_SIZE:7:0:55000]
[BODY_APPEARANCE_MODIFIER:HEIGHT:75:95:98:100:102:105:125]
[APP_MOD_IMPORTANCE:500]
[BODY_APPEARANCE_MODIFIER:BROADNESS:75:95:98:100:102:105:125]
[APP_MOD_IMPORTANCE:500]
[SET_BP_GROUP:BY_CATEGORY:EYE]
[BP_APPEARANCE_MODIFIER:CLOSE_SET:0:70:90:100:110:130:200]
[APP_MOD_NOUN:eyes:PLURAL]
[BP_APPEARANCE_MODIFIER:DEEP_SET:0:70:90:100:110:130:200]
[APP_MOD_NOUN:eyes:PLURAL]
[BP_APPEARANCE_MODIFIER:ROUND_VS_NARROW:0:70:90:100:110:130:200]
[APP_MOD_NOUN:eyes:PLURAL]
[BP_APPEARANCE_MODIFIER:LARGE_IRIS:25:70:90:100:110:130:200]
[APP_MOD_NOUN:eyes:PLURAL]
[APP_MOD_DESC_RANGE:30:60:90:110:150:190]
[SET_BP_GROUP:BY_CATEGORY:LIP]
[BP_APPEARANCE_MODIFIER:THICKNESS:50:70:90:100:110:130:200]
[APP_MOD_NOUN:lips:PLURAL]
[APP_MOD_DESC_RANGE:55:70:90:110:150:190]
[SET_BP_GROUP:BY_CATEGORY:NOSE]
[BP_APPEARANCE_MODIFIER:BROADNESS:25:70:90:100:110:130:200]
[APP_MOD_DESC_RANGE:30:60:90:110:150:190]
[BP_APPEARANCE_MODIFIER:LENGTH:25:70:90:100:110:130:200]
[APP_MOD_DESC_RANGE:30:60:90:110:150:190]
[BP_APPEARANCE_MODIFIER:UPTURNED:0:70:90:100:110:130:200]
[BP_APPEARANCE_MODIFIER:CONVEX:0:70:90:100:110:130:200]
[APP_MOD_NOUN:nose bridge:SINGULAR]
[SET_BP_GROUP:BY_CATEGORY:EAR]
[BP_APPEARANCE_MODIFIER:SPLAYED_OUT:0:70:90:100:110:130:200]
[APP_MOD_NOUN:ears:PLURAL]
[BP_APPEARANCE_MODIFIER:HANGING_LOBES:0:70:90:100:110:130:200]
[APP_MOD_NOUN:ears:PLURAL]
[BP_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[APP_MOD_IMPORTANCE:700]
[APP_MOD_NOUN:ears:PLURAL]
[APP_MOD_DESC_RANGE:91:94:98:102:106:109]
[BP_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[APP_MOD_IMPORTANCE:700]
[APP_MOD_NOUN:ears:PLURAL]
[APP_MOD_DESC_RANGE:91:94:98:102:106:109]
[SET_BP_GROUP:BY_CATEGORY:TOOTH]
[BP_APPEARANCE_MODIFIER:GAPS:0:70:90:100:110:130:200]
[APP_MOD_NOUN:teeth:PLURAL]
[BP_APPEARANCE_MODIFIER:LENGTH:100:100:100:100:100:100:100] for vampires
[APP_MOD_IMPORTANCE:1000]
[APP_MOD_NOUN:teeth:PLURAL]
[APP_MOD_DESC_RANGE:30:60:90:110:150:190]
[SET_BP_GROUP:BY_CATEGORY:SKULL]
[BP_APPEARANCE_MODIFIER:HIGH_CHEEKBONES:0:70:90:100:110:130:200]
[BP_APPEARANCE_MODIFIER:BROAD_CHIN:0:70:90:100:110:130:200]
[BP_APPEARANCE_MODIFIER:JUTTING_CHIN:0:70:90:100:110:130:200]
[BP_APPEARANCE_MODIFIER:SQUARE_CHIN:0:70:90:100:110:130:200]
[SET_BP_GROUP:BY_CATEGORY:HEAD]
[BP_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[APP_MOD_IMPORTANCE:700]
[APP_MOD_DESC_RANGE:91:94:98:102:106:109]
[BP_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[APP_MOD_IMPORTANCE:700]
[APP_MOD_DESC_RANGE:91:94:98:102:106:109]
[NATURAL_SKILL:SNEAK:16]
[MAXAGE:100:120]
[ATTACK:PUNCH:BODYPART:BY_TYPE:GRASP]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:punch:punches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:3:3]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:4:4]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[ATTACK_FLAG_BAD_MULTIATTACK]
[ATTACK:SCRATCH:CHILD_TISSUE_LAYER_GROUP:BY_TYPE:GRASP:BY_CATEGORY:FINGER:NAIL]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:scratch:scratches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PREPARE_AND_RECOVER:3:3]
[ATTACK_PRIORITY:SECOND]
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_CATEGORY:HEAD:BY_CATEGORY:TOOTH]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:200]
[ATTACK_FLAG_EDGE]
[ATTACK_PREPARE_AND_RECOVER:3:3]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
[BABY:1]
[CHILD:12]
[EQUIPS]
[DIURNAL]
[SMELL_TRIGGER:90]
[HOMEOTHERM:10067]
[APPLY_CREATURE_VARIATION:STANDARD_BIPED_GAITS:900:675:450:225:1900:2900] 39 kph
[APPLY_CREATURE_VARIATION:STANDARD_CLIMBING_GAITS:6561:6115:5683:1755:7456:8567] 5 kph, NO DATA
[APPLY_CREATURE_VARIATION:STANDARD_SWIMMING_GAITS:5341:4723:4112:1254:6433:7900] 7 kph
[APPLY_CREATURE_VARIATION:STANDARD_CRAWLING_GAITS:2990:2257:1525:731:4300:6100] 12 kph, NO DATA
[SWIMS_INNATE]
[MUNDANE]
[CASTE:FEMALE]
[FEMALE]
[MULTIPLE_LITTER_RARE]
[CASTE:MALE]
[MALE]
And it's sleep deprivation thingy.
[INTERACTION:SCP-966]
[I_SOURCE:CREATURE_ACTION]
[IS_NAME:SCP-966 sleep deprivation]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_REQUIRES:INTELLIGENT]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_NAME:SCP-966 sleep deprivation]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_IMMUNE_CREATURE:SCP-966]
[CE_ADD_TAG:NOSLEEP]
[CE_DROWSINESS:SEV:100:PROB:100:START:2400:PEAK:72000:END:144000]
[CE_BLEEDING:SEV:1000:PROB:100:BP:BODY_PART:BRAIN:START:72000:PEAK:72500:END:7300] (to simulate death from sleep deprivation. In the unlikely event of the patient surviving, they'll eventually recover from the syndrome. Happens after 2 ingame months.)