The Patronus Charm - 3rd-level Charm (ritual)
Casting Time: 1 action
Range: 10 feet
Duration: Concentration, up to 1 minute

A Patronus Charm is a special bit of magic that requires a wizard to envision one of their happiest memories while casting the spell. The feeling of happiness must be genuine or strong enough to produce a radiant, ethereal beast, the embodiment of that wizard's positive emotions that serves as their protector.

When you cast this spell, you can choose to conjure an incorporeal or corporeal patronus. If you attempt to cast this spell while frightened or within 60 feet of a dementor, you must make an ability check using your spellcasting ability.

The DC equals 10 + the number of dementors within 60 feet of you. A roll of 19-20 on the die automatically succeeds.

On a success, you cast the spell. On a failure, you can only conjure an incorporeal patronus, but if you fail the check by 5 or more, the spell fails entirely.

At the end of your turn, if you are frightened or there are one or more dementors within 60 feet of you while concentrating on this spell, you must repeat the ability check. On a failure, your patronus vanishes and the spell ends.

When you cast the spell and as a bonus action on subsequent turns, you can move the patronus. The distance you can move it depends on the size of the patronus. A Medium or smaller patronus can move 60 feet, a Large patronus can move 45 feet, and a Huge patronus can move 30 feet. A patronus ignores objects and terrain. In addition, the Patronus can enter a creature's space and stop there.

At any time during the patronus's movement, you can direct the patronus to charge into a dementor (or other affectable creature) within 5 feet of it.

Make a melee spell attack for the patronus using your spell attack modifier. On a hit, the target takes 5d10 radiant damage and the patronus pushes the target up to 5 feet plus a number of feet equal to five times your spellcasting ability modifier. The patronus moves with the target to remain within 5 feet of it and does not use any movement to do so.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d10 and the radius of the dim light increases by 5 feet for each slot level above 3rd.

Highly Advanced Magic
The Patronus Charm is regarded as a difficult spell partly because the caster must relive a truly joyful memory when casting. This spell's ability check is unusual, but represents the challenge of focusing on a suitable memory in a stressful situation.
Remember that a frightened creature has disadvantage on ability checks, including this one.
Your DM can allow you to spend Inspiration to automatically succeed on this check.
Your character might bring forth a powerful childhood memory in their time of need.

A patronus sheds light in a radius around it. You and friendly creatures can't be frightened while in your patronus's light. A dementor that starts its turn within this light or enters the light for the first time on a turn must succeed on a Wisdom saving throw or become frightened of the patronus until the start of its next turn (this fear ignores any immunity to the frightened condition). While frightened, the dementor must take the Dash action on its turn and move away from the patronus by the safest available route until it leaves the light.

Incorporeal Patronus.
Your patronus takes the form of a 5-foot burst of glowing mist in an unoccupied space adjacent to you. The patronus doesn't fill its space. It sheds bright light in its space and dim light in a 5-foot radius. On subsequent turns, you can move the patronus to another space adjacent to you as a bonus action. Additionally, you can move it as a reaction to a dementor moving within 10 feet of you.

Corporeal Patronus.
Your patronus takes the form of a wispy silver animal in an unoccupied space that you can see within range. The patronus doesn't fill its space. It has no hit points and is immune to all damage, but can be targeted by another spell that affects magic, such as finite incantatem.

The patronus sheds bright light in a 10-foot radius and dim light for an additional 10 feet.

More than Dementors
The Patronus Charm is most famous against dementors, but it can be a potent protector against other dark creatures in the Wizarding World.
Only dementors and lethifolds have been confirmed to be affected by a patronus. At the HM's discretion, your patronus can interact with or harm creatures like banshees, inferi, or obscuri. If your HM decides a creature can be affected by a patronus, the creature is susceptible to the patronus's fear effect, attacks and damage.
Even if a patronus is unable to harm a particular kind of creature, its soothing presence can still protect you by preventing fear.

You can determine your Patronus' form by rolling on the Expecto Patronum - Rolling Table. If you are of the Transfiguration School of Magic your Patronus will take the same form as your animagus (see Transfiguration: Choosing an animal)