Mark for Death — ruling 1

That creature’s controller still decides which attacking creature it blocks.

Mark for Death — ruling 2

If the creature can’t block (perhaps because it has become tapped or it doesn’t have flying and all attacking creatures do), then the requirement to block does nothing.

Mark for Death — ruling 3

If there are any costs required for that creature to block (such as the one imposed by War Cadence), the creature’s controller isn’t required to pay those costs. In that case, the requirement to block would do nothing.

Mark for Death : règles, fonctionnement et interactions (MTG)

Mark for Death 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 an opponent controls blocks this turn if able. Untap that creature. Other creatures that player controls can't block this turn.

How it works : Mark for Death

Original English text — Wizards of the Coast / Scryfall

#1January 24, 2013· Wizards of the Coast
That creature’s controller still decides which attacking creature it blocks.
No other card is named in this ruling.
#2January 24, 2013· Wizards of the Coast
If the creature can’t block (perhaps because it has become tapped or it doesn’t have flying and all attacking creatures do), then the requirement to block does nothing.
No other card is named in this ruling.
#3January 24, 2013· Wizards of the Coast
If there are any costs required for that creature to block (such as the one imposed by War Cadence), the creature’s controller isn’t required to pay those costs. In that case, the requirement to block would do nothing.
No other card is named in this ruling.