King played for Everton last season, making 11 appearances but failing to scoreJoshua King's hat-trick helped Watford to a stunning first win under Claudio Ranieri as Everton collapsed in a pulsating Premier League game at Goodison Park.
Tom Davies put the hosts ahead early on but Watford equalised before the break through King from close range.
Former Hornets striker Richarlison put Everton ahead again but Juraj Kucka's header drew Watford level.
King, who didn't score for Everton during his spell there in the first half of 2021, then struck twice inside six minutes before Emmanuel Dennis put the seal on an incredible comeback.
While Watford had never won at Goodison in any competition before this match, Ranieri has happier memories of trips to Everton - this was his fifth consecutive win at the ground.
It is a second successive home defeat for Everton's former Liverpool manager Rafael Benitez and some boos greeted the Spaniard's decision to replace Anthony Gordon with Richarlison in the second half.
He seemed vindicated when the Brazilian struck with a fine diving header just three minutes after coming on, but Benitez then saw his side's lead evaporate in a disastrous final 10 minutes that leaves his side in eighth place.
Watford, having started life under Ranieri with an emphatic defeat against Liverpool, climb the table to 14th - suddenly well clear of the relegation zone.
Hornets sting tentative Toffees
When Davies slid in to convert Demarai Gray's low cross from close range after three minutes it looked like being another difficult day against Merseyside opposition for Ranieri following their 5-0 thrashing at Vicarage Road a week earlier.
But while Everton failed to build on that start and were tentative, Watford took the attack to the home side in defiance of having the lowest number of shots in the Premier League - 76 - prior to this match.
King's equaliser was initially ruled out for offside by referee Graham Scott but after looking at the pitchside monitor he overturned his decision because Gray had just played King onside.
Watford rallied after Richarlison's goal - with Ranieri immediately throwing on Dennis and Joao Pedro - and Kucka got the visitors back on level terms with a powerful header from Cucho Hernandez's corner.
Ranieri's substitutions then sparked the late drama, with Dennis' cross finding King with the time and space to steady himself for his second before the pair linked up again when the Norway striker kept his head again inside the box to claim his hat-trick.
Dennis has enjoyed a good start to his Premier League career, despite Watford's struggles, and the Nigerian claimed his third goal of the season after an assist from Pedro as large sections of the home support headed for the exits.
Line-ups
Everton
Formation 4-4-1-1
1Pickford
23Coleman5Keane22Godfrey12Digne
14Townsend26Davies6Allan24Gordon
11Gray
33Rondón
1Pickford23Coleman5Keane22Godfrey12DigneBooked at 17mins14Townsend26Davies6AllanBooked at 54mins24GordonSubstituted forRicharlisonat 60'minutesBooked at 72mins11GraySubstituted forIwobiat 77'minutes33RondónBooked at 90minsSubstitutes
2Kenny4Holgate7Richarlison15Begovic17Iwobi25Gbamin31Lonergan61DobbinWatford
Formation 4-1-4-1
1Foster
2Ngakia5Troost-Ekong15Cathcart11Masina
19Sissoko
23Sarr18Tufan33Kucka29Hernández Suárez
7King
1Foster2NgakiaSubstituted forNkoulouat 76'minutes5Troost-EkongBooked at 44mins15Cathcart11Masina19Sissoko23SarrSubstituted forDennisat 64'minutesBooked at 70mins18TufanSubstituted forJoão Pedroat 64'minutes33Kucka29Hernández Suárez7KingBooked at 81minsSubstitutes
6Louza8Cleverley10João Pedro12Sema13Nkoulou16Gosling17Fletcher25Dennis26BachmannReferee:Graham ScottAttendance:38,834Live Text
Match ends, Everton 2, Watford 5.
Second Half ends, Everton 2, Watford 5.
Foul by Emmanuel Dennis (Watford).
Ben Godfrey (Everton) wins a free kick in the defensive half.
Salomón Rondón (Everton) is shown the yellow card for a bad foul.
Juraj Kucka (Watford) wins a free kick in the defensive half.
Foul by Salomón Rondón (Everton).
Goal! Everton 2, Watford 5. Emmanuel Dennis (Watford) right footed shot from the centre of the box to the centre of the goal. Assisted by João Pedro.
Ben Foster (Watford) wins a free kick in the defensive half.
Foul by Tom Davies (Everton).
Corner, Everton. Conceded by Adam Masina.
Foul by Juraj Kucka (Watford).
Tom Davies (Everton) wins a free kick on the left wing.
Goal! Everton 2, Watford 4. Joshua King (Watford) left footed shot from the centre of the box to the centre of the goal. Assisted by Emmanuel Dennis.
Attempt blocked. João Pedro (Watford) right footed shot from outside the box is blocked. Assisted by Emmanuel Dennis.
Attempt missed. Cucho Hernández (Watford) right footed shot from outside the box is too high. Assisted by Emmanuel Dennis.
Attempt missed. Lucas Digne (Everton) left footed shot from the left side of the box is too high. Assisted by Richarlison with a cross.
Attempt missed. Juraj Kucka (Watford) header from the centre of the box is too high. Assisted by Cucho Hernández with a cross following a corner.
Corner, Watford. Conceded by Jordan Pickford.
Attempt saved. Emmanuel Dennis (Watford) right footed shot from the centre of the box is saved in the bottom right corner.
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