Harry Porterie's most powerful spell was used by him and his cousin when they met dementors.

Expecto Patronum

The spell of summoning the patron saint is: summoning the guardian. This spell will only work when you concentrate, so when you summon the patron saint, you must try your best to remember some happy things. It can not only summon its patron saint to drive away dementors, but also be used for communication (that's what people in the Order of the Phoenix do because it's safe). The patron saint is generally a silvery white animal, like a ghost in the fog. Its shape varies from person to person, which is related to the wizard's character and physique. The patron saint is not eternal, but will change when he is hit hard and his mood changes greatly. (The sixth patron saint of Tonks)

The trick to using the patron saint spell is to recall happy things, but Ms. Rowling is not the first person to remember happy things as a trick. In Peter Pan, Peter Pan taught children how to fly and recalled happy things.

The patron saint is the power of justice. It is different from death eaters crossing the dark mark, so death eaters have no patron saint. The only death eater with a patron saint is severus snape.

Wizards and their patron saint:

Harry and his father-Buck

Lily potter-Zhangzi

Severus snape-Zhangzi

Hermione otter

Ron.-hounds

Albus dumbledore-Phoenix

Aberforth dumbledore goat

Tonks-Wolf (this is after she fell in love with Lu Bang, but the previous one is unknown)

Lubang-Wolff

Kingsley Shacklebolt-Bobcat

Author weasel-Weasel

Professor McGonagall-tabby cat

Sirius-Dog (Guess)

Ginny Ma (in the movie)

Zhao changwang

moon rabbit

Seymour finnigan-Fox

Ernie Macmillan-boar

Umbridge cat