What CR is Mushroom Droven?

It is built as a CR 2 (450 XP) Small fey at its native Tier 1, sized for a level 1-4 party. The stat block also scales up to Tier 2 (levels 5-10) and Tier 3 (levels 11-15) by swapping in the listed AC, HP, to-hit, and save DC; traits and actions stay identical.

Can Mushroom Droven be a companion or ally?

Yes. Won over with honey and a promise to spare its toadstool rings, a bound droven rides along as a support skirmisher — see the companion block. It snipes with poisoned darts, fades behind smoke as a reaction, and once per short rest exhales a healing spore cloud that mends an ally and lifts fright.

How do I stop the drowsing smoke from wrecking my party?

The smoke only forces a Constitution save when a creature ends its turn inside the cube, so move out of it. Poison resistance or immunity, advantage on Con saves, or simply blinding-fighting around the obscurement all blunt it. Killing or driving off the droven removes the smoke at the start of its next turn.