Abyssal Nocturnus — ruling 1

If your opponent discards any cards during their cleanup step to get down to the maximum hand size, the ability will trigger and players will receive priority. After that cleanup step ends, a new one will begin and Abyssal Nocturnus's effect will end. It won't carry over into the next turn.

Abyssal Nocturnus — ruling 2

If your opponent discards multiple cards to a single effect, this will trigger multiple times.

Abyssal Nocturnus : règles, fonctionnement et interactions (MTG)

Abyssal Nocturnus is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Whenever an opponent discards a card, this creature gets +2/+2 and gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)

How it works : Abyssal Nocturnus

Original English text — Wizards of the Coast / Scryfall

#1February 1, 2006· Wizards of the Coast
If your opponent discards any cards during their cleanup step to get down to the maximum hand size, the ability will trigger and players will receive priority. After that cleanup step ends, a new one will begin and Abyssal Nocturnus's effect will end. It won't carry over into the next turn.
No other card is named in this ruling.
#2February 1, 2006· Wizards of the Coast
If your opponent discards multiple cards to a single effect, this will trigger multiple times.
No other card is named in this ruling.