Markov Crusader — ruling 1

If Markov Crusader loses haste before the declare attackers step on the turn it enters the battlefield, it won't be able to attack. Once you've attacked with Markov Crusader, causing it to lose haste won't remove it from combat.

Markov Crusader : règles, fonctionnement et interactions (MTG)

Markov Crusader 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

Lifelink This creature has haste as long as you control another Vampire.

How it works : Markov Crusader

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2016· Wizards of the Coast
If Markov Crusader loses haste before the declare attackers step on the turn it enters the battlefield, it won't be able to attack. Once you've attacked with Markov Crusader, causing it to lose haste won't remove it from combat.
No other card is named in this ruling.