Devout Lightcaster — ruling 1

Devout Lightcaster's triggered ability isn't optional. When it enters, if you're the only player that controls a black permanent, you'll have to target a black permanent you control.

Devout Lightcaster : règles, fonctionnement et interactions (MTG)

Devout Lightcaster 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

Protection from black When this creature enters, exile target black permanent.

How it works : Devout Lightcaster

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
Devout Lightcaster's triggered ability isn't optional. When it enters, if you're the only player that controls a black permanent, you'll have to target a black permanent you control.
No other card is named in this ruling.