Stench. Constitution Saving Throw: DC 13, any creature that starts its turn in a 5-foot Emanation originating from the blob. Failure: The target has the Poisoned condition until the start of its next turn. Success: The target is immune to this blob's Stench for 1 minute.

Amorphous.  The Undead Blob's amorphous nature makes it resistant to non-magical, non-silvered bludgeoning/slashing/piercing damage.

Multiattack. The blob makes two Horrific Necrosis attacks. It can replace one attack with a Tendril attack.

Tendril. Melee Attack Roll: +6, reach 10 ft. Hit: 6 (1d6 + 3) Slashing damage. If the target isn’t an Undead, it must make a DC 15 Constitution saving throw.   Failure: The target has the Paralyzed condition until the end of its next turn.

Horrific Necrosis. Melee or Ranged Attack Roll: +7, reach 5 ft. or range 120 ft. Hit: 10 (1d10 + 4) Necrotic damage, the target must make a DC 15 Wisdom saving throw.   Failure: The target has the Frightened condition until the end of its next turn.

Spellcasting. The blob casts one of the following spells, requiring no Material components and using Intelligence as the spellcasting ability and it's Spell DC is 15 and Spell Attack is +7:

At Will: Blindness/Deafness, Ray of Sickness

1x/day: Spirit Shroud