Ashling's Prerogative — ruling 1

The "chosen value" is either "odd" or "even." So either creatures with odd mana values enter tapped and creatures with even mana values have haste, or vice versa.

Ashling's Prerogative — ruling 2

If a creature has X in its mana cost, that X is treated as 0 for the purposes of these effects. It doesn't matter what the value of X was while the creature was on the stack.

Ashling's Prerogative : règles, fonctionnement et interactions (MTG)

Ashling's Prerogative 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

As this enchantment enters, choose odd or even. (Zero is even.) Each creature with mana value of the chosen quality has haste. Each creature without mana value of the chosen quality enters tapped.

How it works : Ashling's Prerogative

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2007· Wizards of the Coast
The "chosen value" is either "odd" or "even." So either creatures with odd mana values enter tapped and creatures with even mana values have haste, or vice versa.
No other card is named in this ruling.
#2October 1, 2007· Wizards of the Coast
If a creature has X in its mana cost, that X is treated as 0 for the purposes of these effects. It doesn't matter what the value of X was while the creature was on the stack.
No other card is named in this ruling.