Deadly Designs — ruling 1

You control the last ability of Deadly Designs, no matter who activated the ability to give it its fifth plot counter.

Deadly Designs — ruling 2

You must sacrifice Deadly Designs as its last ability resolves. However, you can choose zero creatures as targets if you want.

Deadly Designs — ruling 3

If the player who controlled Deadly Designs as its last ability triggered doesn't control it as that ability resolves, Deadly Designs won't be sacrificed. The same is true if each of the ability's targets are all illegal by the time the ability resolves. However, if it's still on the battlefield and has five or more plot counters on it, its last ability will immediately trigger again.

Deadly Designs : règles, fonctionnement et interactions (MTG)

Deadly Designs 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

: Put a plot counter on this enchantment. Any player may activate this ability. When there are five or more plot counters on this enchantment, sacrifice it. If you do, destroy up to two target creatures.

How it works : Deadly Designs

Original English text — Wizards of the Coast / Scryfall

#1March 16, 2018· Wizards of the Coast
You control the last ability of Deadly Designs, no matter who activated the ability to give it its fifth plot counter.
No other card is named in this ruling.
#2March 16, 2018· Wizards of the Coast
You must sacrifice Deadly Designs as its last ability resolves. However, you can choose zero creatures as targets if you want.
No other card is named in this ruling.
#3March 16, 2018· Wizards of the Coast
If the player who controlled Deadly Designs as its last ability triggered doesn't control it as that ability resolves, Deadly Designs won't be sacrificed. The same is true if each of the ability's targets are all illegal by the time the ability resolves. However, if it's still on the battlefield and has five or more plot counters on it, its last ability will immediately trigger again.
No other card is named in this ruling.