Elspeth, Undaunted Hero — ruling 1

The value of X is determined only as Elspeth’s last ability resolves. Once that happens, the value of X won’t change later in the turn even if your devotion to white changes.

Elspeth, Undaunted Hero — ruling 2

Elspeth’s last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t get +X/+X or gain flying.

Elspeth, Undaunted Hero — ruling 3

Colorless and generic mana symbols ({C}, {0}, {1}, {2}, {X}, and so on) in mana costs of permanents you control don’t count toward your devotion to any color.

Elspeth, Undaunted Hero — ruling 4

Mana symbols in the text boxes of permanents you control don’t count toward your devotion to any color.

Elspeth, Undaunted Hero — ruling 5

Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).

Elspeth, Undaunted Hero — ruling 6

If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it’s still on the battlefield at that time.

Elspeth, Undaunted Hero — ruling 7

If you put an Aura on an opponent’s permanent, you still control the Aura, and mana symbols in its mana cost count towards your devotion.

Elspeth, Undaunted Hero : règles, fonctionnement et interactions (MTG)

Elspeth, Undaunted Hero 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

+2: Put a +1/+1 counter on each of up to two target creatures. −2: Search your library and/or graveyard for a card named Sunlit Hoplite and put it onto the battlefield. If you search your library this way, shuffle. −8: Until end of turn, creatures you control gain flying and get +X/+X, where X is your devotion to white.

How it works : Elspeth, Undaunted Hero

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2020· Wizards of the Coast
The value of X is determined only as Elspeth’s last ability resolves. Once that happens, the value of X won’t change later in the turn even if your devotion to white changes.
No other card is named in this ruling.
#2January 24, 2020· Wizards of the Coast
Elspeth’s last ability affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won’t get +X/+X or gain flying.
No other card is named in this ruling.
#3January 24, 2020· Wizards of the Coast
Colorless and generic mana symbols (, , , , , and so on) in mana costs of permanents you control don’t count toward your devotion to any color.
No other card is named in this ruling.
#4January 24, 2020· Wizards of the Coast
Mana symbols in the text boxes of permanents you control don’t count toward your devotion to any color.
No other card is named in this ruling.
#5January 24, 2020· Wizards of the Coast
Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).
No other card is named in this ruling.
#6January 24, 2020· Wizards of the Coast
If an activated ability or triggered ability has an effect that depends on your devotion to a color, you count the number of mana symbols of that color among the mana costs of permanents you control as the ability resolves. The permanent with that ability will be counted if it’s still on the battlefield at that time.
No other card is named in this ruling.
#7January 24, 2020· Wizards of the Coast
If you put an Aura on an opponent’s permanent, you still control the Aura, and mana symbols in its mana cost count towards your devotion.
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Elspeth, Undaunted Hero.

Elspeth's Devotee

When this creature enters, you may search your library and/or graveyard for a card named Elspeth, Undaunted Hero, reveal it, and put it into your hand. If you search your library this way, shuffle.