Adrenal-shrooms.

When Adrenal-shrooms are attached to a living creature, they steal nutrients from the host and store it. When the host is close to death or manually squeezed, it provides a short 'buff' that allows both the host and the shrooms to stay alive. If these mushrooms are to be harmed it inflicts heavy pain and bleeding upon the host, as they are connected to the bloodstream.

In gameplay, this buff provides the user with better rolls on ability and attack rolls (+3), increase in hit-points (+4) and more walk speed (+5). These wear off after 3 turns, putting the user in level 3 exhaustion for 1 turn. This item also has a passive effect that requires the host to consume more nutrition or risk the shrooms dying. Attacking the shrooms has disadvantage due to their small size. If the shrooms are harmed while attached, the host is poisoned for 1 turn and loses 2 HP. 

Notes: Exhaustion, Moderately inspired by Water-shrooms from Made in Abyss., Druid or Ranger, takes 8 hours to fully attach., Buff, Combat