Egill aka the "Taciturn Druid" was a druid who lived in a grotto under the sacred tree Gedyneith on Ard Skellig. By 1272 he hadn't spoken in three years as he'd taken a vow of silence.
If Geralt "cures" him: Geralt, tricked into thinking it wasn't selective mutism, did all he could to scare or irritate the druid until, out of anger, Egill yelled at the witcher before stopping mid-sentence, realizing he'd broken his vow. He then told Geralt and the trickster off.