Witchstalker — ruling 1

The counter will be put on Witchstalker before the spell cast by an opponent resolves.

Witchstalker — ruling 2

If an opponent casts a copy of a card that’s blue or black during your turn (as opposed to putting a copy of such a spell directly on the stack), the ability will also trigger. Check for the word “cast” in the effect that creates the copy.

Witchstalker : règles, fonctionnement et interactions (MTG)

Witchstalker 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

Hexproof (This creature can't be the target of spells or abilities your opponents control.) Whenever an opponent casts a blue or black spell during your turn, put a +1/+1 counter on this creature.

How it works : Witchstalker

Original English text — Wizards of the Coast / Scryfall

#1July 1, 2013· Wizards of the Coast
The counter will be put on Witchstalker before the spell cast by an opponent resolves.
No other card is named in this ruling.
#2July 1, 2013· Wizards of the Coast
If an opponent casts a copy of a card that’s blue or black during your turn (as opposed to putting a copy of such a spell directly on the stack), the ability will also trigger. Check for the word “cast” in the effect that creates the copy.
No other card is named in this ruling.