Xill

Xill are strange extraplanar creatures, inhabiting the Ethereal Plane just aside the world, or the edges of the Overworld in the Dragon Empire. The largest faction of xill are vicious raiders, hunting victims to expand their numbers as much as possible. All xill are capable of producing eggs, and can even self fertilize to perform reproduction runs on their own. When encountering prey, xill will attempt to use their paralytic bite to render their victim helpless, then inject its eggs into the creature’s body via an ovipositor located behind the xill’s mandibles.

Generally, once their eggs have been implanted the xill will retreat from battle, seeking to return home to rest, recover, and prepare for their next raid, leaving their eggs to incubate and hatch. Young xill instinctively know how to planewalk, attempting to hide and make their way to the Ethereal, where they can grow. Young xill are collected by adults wherever they are found and given training and indoctrination into the culture of their rescuers. Those smaller groups that don’t believe in reckless and cruel expansion will try to train the young to that view, but they are constantly under assault from the much larger faction of expansionists who see all other creatures as nothing more than incubators, and these kinder xill as weaklings and traitors.

Xill track back all the way to the Fiend Folio from 1981, and appeared in both the 3.5 Monster Manual and Pathfinder Bestiary. They did appear in one 5e supplement, but I wanted to get my hands on them as well to make my own version. This post came out a week ago on my Patreon. If you want to get access to all my monster conversions early, as well as access to my premade adventures and other material I’m working on, consider backing me there!

5th Edition

Xill
Medium humanoid, usually lawful evil
Armor Class 18 (natural armor, shield)
Hit Points 91 (14d8 + 28)
Speed 40 ft.
Str 17 (+3) Dex 18 (+4) Con 14 (+2) Int 15 (+2) Wis 12 (+1) Cha 11 (+0)
Saving Throws Dex +7, Int +5, Wis +4
Skills Acrobatics +7, Arcana +5, Deception +3, Intimidation +3, Perception +4, Stealth +7
Senses darkvision 60 ft., passive Perception 14
Languages Common, Infernal
Challenge 6 (2300 XP)
Actions
Multiattack. The xill makes four attacks, only one of which can be with its Bite. If it hits a single target with two or more claw attacks, it grapples that target (escape DC 13).
Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (1d4+4) piercing damage and the target must make a DC 13 Constitution saving throw. If the saving throw fails by 5 or more, the target is paralyzed for 1d4 hours. Otherwise, a creature that fails the save feels its limbs going numb and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming paralyzed for 1d4 hours on a failure, or ending the effect on a success. A paralyzed creature can repeat the save at the end of each hour, ending the effect on itself on a success.
Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 6 (1d4+4) slashing damage.
Shortsword. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 7 (1d6+4) piercing damage.
Implant Eggs (1/Day). The xill can lay 2d6 eggs in an incapacitated creature, or a creature it has grappled. The eggs hatch in 24 hours, at which point they begin devouring the host from within. Each hour, the target has its hit point maximum reduced by 5 (2d4) per young. If the target’s hit point maximum drops to 0 as a result of this, the target dies. The young then emerge from the corpse. The young are Tiny fiends with AC 6, 1 hit point, a 2 (-4) in every ability score, a speed of 10 feet, and the Planewalk action.
The eggs or active young can be removed with a lesser restoration spell or similar magic. Alternatively, a creature can spend 10 minutes and deal 5 (2d4) damage to the host to make a DC 16 Wisdom (Medicine) check, removing a single egg or active young on a success.
Planewalk. The xill enters the Material Plane from the Ethereal Plane. Alternatively, the xill can use its action and bonus action for three rounds in a row to enter the Ethereal Plane from the Material Plane. The xill’s attempt to enter the Ethereal Plane is interrupted if it moves or if its concentration is broken (as if concentrating on a spell), in which case it must restart this attempt. After it takes the first round of actions, attacks against the xill have disadvantage.

13th Age

Xill 
3rd level spoiler [humanoid] 
Initiative: +9
Claws and Blades +8 vs. AC (3 attacks) – 2 damage.
Dual Hits: If the xill hits a single target with two or more claws and blades attacks, it can make a paralyzing bite attack as a free action against that target.
[Special Trigger] Paralyzing Bite +8 vs. AC – 2 damage and the target is dazed (save ends).
Natural 16+: The target is weakened instead of dazed (save ends).
First Failed Save: The target is stuck as long as it is dazed or weakened.
Second Failed Save: The target is stunned (save ends).
Implant Eggs +8 vs. PD (one weakened or stunned enemy) – The target must start making last gasp saves. With each failed save, it takes 5 damage as xill young start devouring it from the inside out. On the fourth failed save, the target dies and 2d6 xill young burst out of its corpse. Each xill young has all defenses 10, 1 hit point, and the planewalk action. Even if the battle ends, the target must continue making these saves until it succeeds or it accumulates 4 failures.
Limited Use: 1/battle, recharges if the attack misses.
Planewalk: The xill can spend a standard action to attempt a hard save (16+). On a success, the xill can teleport to another plane of existence, leaving the battle. If all living xill in the battle have successfully used their implant eggs attack, this becomes an easy save (6+).
AC 20 PD 18 MD 14 HP 32