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Williams' appearance came after Eilish and her sibling Finneas played "Your Power" from her latest collection, with the kin duetting on acoustic guitar.
There was an interruption after the tune finished, in which expectation swirled into the atmosphere and Eilish sat down on a stool to present her visitor.
Goodness, look! An unfilled seat. Who's that for ?" Eilish said with a wicked look.
Everyone, could we at any point if it's not too much trouble, welcome my companion Hayley Williams.
The two hopped and embraced as Williams tended to the group. "This is my most memorable Coachella, hold up," Williams said.
We sound great," Williams said before Eilish added her unmistakable murmur style tone to the subsequent section.
Finneas and another guitar player, Williams and Eilish conveyed an acoustic interpretation of the pop-punk tune.
Last weekend, Eilish brought out Damon Albarn of Gorillaz notoriety to perform "Feel Good Inc" alongside Posdnuos of De La Soul.