race features:
Darkvision. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can't discern color in darkness, only shades of gray.
Natural Armor. Due to your gelatinous body you are ill-suited to wear armor. Your thick skin provides ample protection, giving you an AC equal to 12 + your constitution modifier. You gain no benefit from wearing armor, but if you are using a shield, you can apply the shield's bonus as normal.
Safe Keeping. Tar can store objects within themselves for safe keeping. The items in question mustn't extend it's own shape and cannot be forced in quickly. In combat it would take approximately two turns and they must hold concentration. Same to retrieve an item.
Toxic Vomit. You know the cantrip acid splash.
Familiar Scent. Tar can smell diseased and poisonous items and creatures. Including their own kind.
Ooze Nature. You can eat, sleep, and breath but it is unnecessary. When you do take a long rest, you must spend at least six hours in an inactive, motionless state, rather than sleeping. In this state, you appear inert, but it doesn't render you unconscious, and you can see and hear as normal.
Resistances and Immunities. You are resistant to acid damage and immune to poison and disease.
Dehydration-Prone. You are composed of a large amount of viscous liquid and as such, you fare poorly in the burning environments. You have disadvantage on saving throws against Extreme Heat.
Shattered Mask. If a Tar's mask is struck directly with enough damage to break, it will enter the frightened condition trying to get to a safe place to regenerate it.
Tar are primarily, as their name suggests, a thick black substance that can take most any form whether it be humanoid or more monstrous. They are comprised mainly of an inner toxic sludge then an outer layer like a film or skin to retain shape. Each Tar also has a unique mask they grow themselves. The masks can be made of most anything available for them to convert.