Sources
CR
Type
Size
Alignment
Movement

Gaj

Large Aberration, Neutral Evil

Armor Class 16 (natural armor)
Hit Points 75 (10d10 + 20)
Speed 30 ft., climb 30 ft.
STR
16
+3
DEX
10
+0
CON
15
+2
INT
12
+1
WIS
15
+2
CHA
7
-2
Skills Perception +6, Stealth +4
Senses darkvision 60 ft., passive Perception 16
Languages understands all languages but can't speak
Challenge 4 (1,100 XP)

Actions

Multiattack. The gaj makes one Mandibles attack and uses Mind-Probing Antennae or Paralyze (if available).

Mandibles. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit:10 (2d6 + 3) slashing damage, and the target is grappled (escape DC 11). Until the grapple ends, the target takes 10 (2d6 + 3) slashing damage at the start of each of the gaj's turns. While it is grappling a creature, the gaj can't use its mandibles to attack other creatures.

Mind-Probing Antennae. The gaj targets one creature grappled by it. The target must make a DC 12 Wisdom saving throw. On a failed save, the target takes 16 (3d10) psychic damage, and the gaj magically pulls one piece of information from the target's mind that the gaj wants to know. On a successful save, the target takes half as much damage, and the gaj learns nothing.

Paralyze (Recharge). The gaj magically targets one creature it can see within 60 feet of itself. The target must succeed on a DC 12 Wisdom saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Lore

Gaj are hideous hunters that prey on other intelligent life forms. They crawl on six insectile legs and attack with their mandibles. They ambush prey by burying themselves under sand or silt and lying in wait, lurking in dark caves, or perching on natural stone ledges, where they blend in with their surroundings. Gaj have no language, but they have the magical ability to understand the speech of other creatures.