Nymph\Imago (concept)

We all know nymphs as mythical creatures. In a lot of fantasy settings, nymphs are the beautiful women, water spirits, fey, that live near\in the bodies of water and have various abilities matching the universe.
But a real nymph (called that after the mythical creatures) is one of the stages of an insect's development. There are two cycles, a full one (egg-larva-cocoon-imago, example — butterfly) and a short one (agg-nymph-imago, example — mosquito), where the nymph lives and grows in water, and imago — the adult final form — flies up from it without the cocoon metamorphosis stage.

So what if Nymphs are… the nymphs?

Near the pool of water you can find a group of nymphs. Beautiful androgynous water creatures. They don’t leave water for long, get empowered while on a wet surface, and weaken, spending a round out of it. Originally neutral, they don’t attack you on sight, they can even talk to you. They are quite curious about the outer world, because they cannot leave.
They are protecting a clutch — dozens of eggs.
Upon being attacked or otherwise triggered, they fight furiously, using water and poison-based powers. After half of them die, they summon an Imago — the adult creature. Big, flying, still somewhat humanoid and beautiful, but infinitely more dangerous.

Imago can be met through dialogue if you manage to befriend the nymphs and maybe complete some quest for them. In that case, Imago will not be hostile.

Imago has a lot of HP, “vampiric” abilities (healing by successfully biting the creature), can fly, is weak to fire damage, and resistant\immune to poison. Mostly using spell-like abilities or just spells. 

Upon crashing one of the eggs you see a body of a nymph. They wake up, but not instantly. (related dialogue)