Was your ranger wearing the Abyss Beckoner gloves?
Abyss Beckoners (Found in the back of the Zhentarim cave at Waukeen's Rest)
Very Rare
"Demonspirit Aura: The wearer's summoned creature has resistance to all damage except Psychic damage.
At the start of the summoned creature's turn, it must succeed a Wisdom Saving Throw or be driven Mad."