Kjeldoran Frostbeast — ruling 1

Kjeldoran Frostbeast’s ability triggers only if it’s still on the battlefield when the end of combat step begins (after the combat damage step). For example, if it blocks a 7/7 creature and is destroyed, its ability won’t trigger at all. The 7/7 will remain on the battlefield.

Kjeldoran Frostbeast — ruling 2

Its ability will destroy all creatures it’s currently blocking or blocked by, not necessarily the creatures it had blocked or become blocked by during that combat phase. For example, if Kjeldoran Frostbeast is dealt lethal combat damage and regenerates, it’s removed from combat. Its ability will trigger at end of combat, but since it’s no longer blocking or blocked by any creatures, the ability won’t do anything.

Kjeldoran Frostbeast : règles, fonctionnement et interactions (MTG)

Kjeldoran Frostbeast 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

At end of combat, destroy all creatures blocking or blocked by this creature.

How it works : Kjeldoran Frostbeast

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
Kjeldoran Frostbeast’s ability triggers only if it’s still on the battlefield when the end of combat step begins (after the combat damage step). For example, if it blocks a 7/7 creature and is destroyed, its ability won’t trigger at all. The 7/7 will remain on the battlefield.
No other card is named in this ruling.
#2October 1, 2009· Wizards of the Coast
Its ability will destroy all creatures it’s currently blocking or blocked by, not necessarily the creatures it had blocked or become blocked by during that combat phase. For example, if Kjeldoran Frostbeast is dealt lethal combat damage and regenerates, it’s removed from combat. Its ability will trigger at end of combat, but since it’s no longer blocking or blocked by any creatures, the ability won’t do anything.
No other card is named in this ruling.