This is illustration for an old fairy tale by a Finnish author Anni Swan. I like her stories, they're whimsical and lush, but usually with a dark twist to them.
This story is about a hidden island that becomes the secret playground of the merpeople during the night.
"[...] nyt saapuivat vedenkuningas ja hänen ihana tyttärensä. Kultaisia portaita myöten he nousivat käsitysten veden pinnalle, ja kunnioittaen väistyivät aallot heidän tieltään. Vedenkuninkaan olkapäillä liehui merivihreä viitta, ja hänen pitkä partansa ulottui polviin, ja päässä hänellä oli kultakruunu. Vedenkuninkaan tytär oli kauniimpi kaikkia vedenneitosia, ja sen vuoksi kutsuttiin häntä Merihelmeksi. Hänen valkoinen pukunsa oli meren vaahdosta kudottu, ja kiharoissaan hänellä oli säteileviä helmiä." (Anni Swan: "Aaltojen salaisuus")
Edit: On public demand -- a translation of the Finnish quote. I couldn't find any official translation but here's my own attempt:
"Now arrived the Sea King and his lovely daughter. Hand in hand they walked up the golden stairs to the surface of the water, and the waves gave them way with respect. A sea green cloack was flowing on the King's shoulders, and his long beard reached to the knees, and he wore a crown of gold. The Sea King's daughter was most beautiful of all the mermaids, and therefore called the Sea Pearl. Her white dress had been woven from the sea foam, and gems glimmered on her curls." (Anni Swan, Fairy Tales: The Secret of the Waves)
This is acutally pretty awesome but i feel like the daughter is too small in comparision to the kin and the king himself isn't majestic enough(he's too much like a Gandalf the Grey).Hope this helps