Halfdane — ruling 1

Note that the duration of Halfdane's ability is until the end of your next upkeep. If Halfdane's ability doesn't get a chance to resolve (because there are no legal targets when it triggers, or because it's countered), then the effect from the previous turn's ability will continue to work through the rest of that upkeep step. Then that effect will wear off, and Halfdane will revert to being 3/3, its printed power and toughness (unless some other effect is modifying what its power and toughness is).

Halfdane : règles, fonctionnement et interactions (MTG)

Halfdane 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 the beginning of your upkeep, Halfdane's base power and toughness become equal to the power and toughness of target creature other than Halfdane until the end of your next upkeep.

How it works : Halfdane

Original English text — Wizards of the Coast / Scryfall

#1October 1, 2009· Wizards of the Coast
Note that the duration of Halfdane's ability is until the end of your next upkeep. If Halfdane's ability doesn't get a chance to resolve (because there are no legal targets when it triggers, or because it's countered), then the effect from the previous turn's ability will continue to work through the rest of that upkeep step. Then that effect will wear off, and Halfdane will revert to being 3/3, its printed power and toughness (unless some other effect is modifying what its power and toughness is).
No other card is named in this ruling.