Vaevictis Asmadi, the Dire — ruling 1

You must target one of your permanents to sacrifice as well.

Vaevictis Asmadi, the Dire — ruling 2

Vaevictis's triggered ability resolves during the declare attackers step, before blockers are chosen. A creature targeted by its ability won't be around to block, and creatures put onto the battlefield with its ability may be able to do so.

Vaevictis Asmadi, the Dire — ruling 3

If any of the target permanents aren't sacrificed (most likely because one of them became an illegal target, such as by gaining hexproof), that permanent's controller won't reveal the top card of their library or put it onto the battlefield.

Vaevictis Asmadi, the Dire — ruling 4

If any player controls no permanents, Vaevictis's ability won't have sufficient legal targets, so it's removed from the stack without any effect.

Vaevictis Asmadi, the Dire : règles, fonctionnement et interactions (MTG)

Vaevictis Asmadi, the Dire 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

Flying Whenever Vaevictis Asmadi attacks, for each player, choose target permanent that player controls. Those players sacrifice those permanents. Each player who sacrificed a permanent this way reveals the top card of their library, then puts it onto the battlefield if it's a permanent card.

How it works : Vaevictis Asmadi, the Dire

Original English text — Wizards of the Coast / Scryfall

#1July 13, 2018· Wizards of the Coast
You must target one of your permanents to sacrifice as well.
No other card is named in this ruling.
#2July 13, 2018· Wizards of the Coast
Vaevictis's triggered ability resolves during the declare attackers step, before blockers are chosen. A creature targeted by its ability won't be around to block, and creatures put onto the battlefield with its ability may be able to do so.
No other card is named in this ruling.
#3July 13, 2018· Wizards of the Coast
If any of the target permanents aren't sacrificed (most likely because one of them became an illegal target, such as by gaining hexproof), that permanent's controller won't reveal the top card of their library or put it onto the battlefield.
No other card is named in this ruling.
#4July 13, 2018· Wizards of the Coast
If any player controls no permanents, Vaevictis's ability won't have sufficient legal targets, so it's removed from the stack without any effect.
No other card is named in this ruling.