Xathrid Slyblade 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

Hexproof (This creature can't be the target of spells or abilities your opponents control.) : Until end of turn, this creature loses hexproof and gains first strike and deathtouch. (It deals combat damage before creatures without first strike. Any amount of damage it deals to a creature is enough to destroy it.)