Stone Giant — ruling 1

To work as an evasion ability, an attacking creature must already have flying when the declare blockers step begins. Once a creature has become blocked, giving it flying won’t change that.

Stone Giant — ruling 2

If Stone Giant’s ability is activated during a turn’s end step, the delayed triggered ability won’t trigger until the beginning of the next turn’s end step. The targeted creature will be destroyed at that time.

Stone Giant — ruling 3

When the delayed triggered ability resolves, the targeted creature is destroyed, even if it’s no longer a creature, no longer under your control, or no longer has toughness less than Stone Giant’s power at that time.

Stone Giant — ruling 4

If the Giant’s power and/or toughness change so that its toughness is less than its power, you can have the ability target the Giant itself.

Stone Giant : règles, fonctionnement et interactions (MTG)

Stone Giant 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

: Target creature you control with toughness less than this creature's power gains flying until end of turn. Destroy that creature at the beginning of the next end step.

How it works : Stone Giant

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
To work as an evasion ability, an attacking creature must already have flying when the declare blockers step begins. Once a creature has become blocked, giving it flying won’t change that.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
If Stone Giant’s ability is activated during a turn’s end step, the delayed triggered ability won’t trigger until the beginning of the next turn’s end step. The targeted creature will be destroyed at that time.
No other card is named in this ruling.
#3October 1, 2009· Wizards of the Coast
When the delayed triggered ability resolves, the targeted creature is destroyed, even if it’s no longer a creature, no longer under your control, or no longer has toughness less than Stone Giant’s power at that time.
No other card is named in this ruling.
#4August 1, 2008· Wizards of the Coast
If the Giant’s power and/or toughness change so that its toughness is less than its power, you can have the ability target the Giant itself.
No other card is named in this ruling.