Sentinels

Sentinels are linked to fire at the same time at your exact location. If a Sentinel is onscreen then you will be briefly blinded when it fires.

Sentinels fire "deathballs" which will instantly kill you.

Deathballs will destroy anything fired at them. A good way to kill Sentinels is to use the Double Jump Cannon below them.

 

The secret item, Dimensional Flux Shields, will stop deathballs from instantly killing you, but only if your shield level is high enough.