Symbionts are slimey creatures that cannot be easily hit. Increase your Dexterity by 2 and your Constitution by 1.

When you´re without a Host, you can obviously not do much. You´re slow and with weaknesses. But you can take an Host. This Host can be a NPC or a Player. If the Targeted Host doesn´t want to be a Host make both a Constitution Saving throw. If the targeted creature throw higher you cannot take the Creature as an Host. If you already taken the Host and he tries to resist you, make both an Wisdom Saving Throw. If the Host trow higher you lose the Host. The already overtaken Host can only resist if he is charmed by an Enemy or in a short or long rest.

Fire and Thunder Damage are very dangerous for your Body. Both Damage Types make double the normal damage you get from the Attack.

You and you´re Host are one Being. In an encounter your both act in the same turn. While you have an Host, you get a Bonus action. Your skill modifiers get added together. And your HP are now the temporary Hit Points of your Host. If you and your Host get attacked you take the Damage. If you´re Unconscious you lose the Host and he loses your Powers.

Copy at the 2th, 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th and 20th Level permanently one random racial trait or feat of your Host (feats/traits that would give you immunity or resistens against thunder or fire damage would instead reduce your doubled damage take to the normal damage you would taken). If you play a magic casting Class you can instead chose to copy one random spell of your Host (If you already have the spell you get, copy instead one random trait/feat from your Host)

Note:  You can additionally copy one of the personality traits from the Host (Only for roleplayers).