Spellcasting. The archdruid is an 18th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 19, +11 to hit with spell attacks). It has the following druid spells prepared:   Cantrips (at will): druidcraft, guidance, produce flame, shape water   1st level (4 slots): cure wounds, entangle, faerie fire, speak with animals   2nd level (3 slots): animal messenger, beast sense, lesser restoration   3rd level (3 slots): call lightning, conjure animals, meld into stone   4th level (3 slots): conjure woodland beings, dominate beast, grasping vine, wall of fire   5th level (3 slots): mass cure wounds, scrying, nature's wrath   6th level (1 slot): druid grove, heal, heroes' feast   7th level (1 slot): regenerate   8th level (1 slot): control weather   9th level (1 slot): foresight

Amphibious. The hydra can breathe air and water   Multiple Heads: The hydra has five heads. While it has more than one head, the hydra has advantage on saving throws against being blinded, charmed, deafened, frightened, stunned, and knocked unconscious. Whenever the hydra takes 25 or more damage in a single turn, one of its heads dies. If all its heads die, the hydra dies.   At the end of its turn, it grows two heads for each of its heads that died since its last turn, unless it has taken fire damage since its last turn. The hydra regains 10 hit points for each head regrown in this way.   Reactive Heads: For each head the hydra has beyond one, it gets an extra reaction that can be used only for opportunity attacks.   Wakeful: While the hydra sleeps, at least one of its heads is awake.