im using this 100%

the grapple only works on huge targets because using it against medium creatures is redundant because the grappled condition already reduces speed to zero

• Hit Points: Based on CON, the baby's hit dice are (4d4 - 16). The average hit points for 4d4 hit dice is 10 (2.5 x 4) so the average would be -6. It is common for undead to have above average CON values. What about reducing hit dice to 1d4 and increasing CON to 7 (-2)? Average hp would then be 1 (0.5 rounded up).
• Speed: The boost basically has no effect unless the baby is fleeing. If it starts its turn within 10 ft., it has enough movement to attack. If its speed becomes 50 ft, then in reality, its speed becomes 20 ft. to reach the target. With INT 3, the baby isn't smart enough to move tactically so it won't use its new 50 ft. movement to maneuver to best use Explode. I would recommend that instead of the boost, use the Aggressive feature from orc and maybe pump it up so the baby gains 15 ft. of movement. Also, is the baby walking or crawling? If crawling, it could suffer from the prone condition.
• STR: I don't think such a weak creature would force a DC 12 check on grapple. Instead, how about STR 10?
• CON: See Hit Points above.
• Damage Immunities: Since the baby is undead, poison should also be listed.
• Condition Immunities: Poisoned should probably be added.
• Grapple: The target of the grapple should be Man-sized or smaller since the baby is Small. (See Grapple action in PH.) Usually, grapple attacks involve an attack roll and then provide an escape DC. (The escape DC can be rolled with STR or DEX depending on character preference.) Immoveable is not a game condition. Normally, you would add "The creature's speed becomes 0, and it can't benefit from any bonus to its speed." However, this is part of the grappled condition so just one baby successfully grappling a target reduces the target's speed. Was the intent that 5 or more babies attempting to grapple a target automatically succeed (i.e., 4 babies failed to grapple but the 5th auto succeeds on the attack)? If this grapple is changed to an attack, then a baby could take the Help action to allow another baby advantage to the attack roll. Perhaps this covers your intent?
• Explode: Legendary Actions are strange to see on a CR 0 creature. It is also strange to not allow saving throws for an area attack. The percentage of hit points lost when grappled is not really done in 5e as well as the different damage dice for the range. (Much easier on the DM to roll once for all damage.) The name of the attack implies that the baby is destroyed by the action, but isn't clear. Instead of the attack automatically dealing damage, how about simplifying and allowing a saving throw? For example, 13 (3d8) necrotic damage in a 10 ft. radius. Target can roll a DEX save for half damage. If grappled by the exploding baby, the target automatically fails the saving throw.

Due to the Explode damage, CR may be closer to 1/8 I think. (I doubt 1/4 but possible. It is definitely less than the Gas Spore's CR of 1/2.)

I'm using this in a trap where my players are trapped in a large room with a summoning circle surrounded by 30 runes, one rune glows every second in a clockwise manner and to prevent the circle from glowing and "summoning something bad" they have to put one drop of blood in the centre of the circle in the process summoning one of these. In actuality if they let it get to 30 seconds they get released from the room and don't have to deal with exploding shadow babies

Who is mother of darkness?

This

Same here this is so amazing T-T

How is this low rated this is amazing

goofy

who saw the xp to 3 video

Baby of darkness is perhaps the greatest homebrew monster in all of DND history.

This is truly the pinnacle of creativity and ingenuity within the field of game design.

I rate this:
10 undead babies / 10

Can you put a saddle on them to benefit from the mach speed?

Quote from ongothefighter >>

who saw the xp to 3 video

 I did, and his reaction was hilarious.

Absolutely incredible.

Quote from ongothefighter >>

who saw the xp to 3 video

 Yeah...

I made this so my BBEG can spawn them and my team hates me only due to the fact that my BBEG spawned 12 per action and has two actions making it spawn 24 each turn it had and I have only four players. That means there is enough to make them not able to move in one turn and kill them the next turn.

this... This is the best thing ever

Quote from ongothefighter >>

who saw the xp to 3 video

 That's why I'm here

Baby of darkness strat:

Run an encounter with a few vampires with a ton of babies of darkness. Whenever the vampires get low, the babies use their legendary actions to explode, healing the vampires since they heal from necrotic damage.

I actually used this in a dungeon in which my players had to defeat 10 Babies of darkness and every round 1 more Baby of darkness would spawn.