Erratic Cyclops — ruling 1

If a spell has {X} in its mana cost, include the value chosen for that X when determining the mana value of that spell.

Erratic Cyclops — ruling 2

Erratic Cyclops’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Erratic Cyclops : règles, fonctionnement et interactions (MTG)

Erratic Cyclops 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

Trample Whenever you cast an instant or sorcery spell, this creature gets +X/+0 until end of turn, where X is that spell's mana value.

How it works : Erratic Cyclops

Original English text — Wizards of the Coast / Scryfall

#1October 5, 2018· Wizards of the Coast
If a spell has in its mana cost, include the value chosen for that X when determining the mana value of that spell.
No other card is named in this ruling.
#2October 5, 2018· Wizards of the Coast
Erratic Cyclops’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
No other card is named in this ruling.