Radiant Oath

The Keepers of the Veil are an order of holy knights with a history dedicated to the destruction of undead, specifically free-willed spirits that should have remained on their own side of the great veil of death. The order is almost eight hundred years old.

— Ptolus 5e p117

Purified Life

The Knights of the Veil endure a rigorous initiation schedule meant to test their mind, their body and their soul. This is to ensure they have the mettle required to face the unholy unliving things that they oppose.

Knights Feast

The ceremony begins with a feast for the new knights joining. Where most of the new candidates relax themselves a bit the reality if the senior and veteran members are using this last moment for a reason to cast doubt on the quality. The feast is also a weaker version of Heroes Feast.

Lesser Feast

A creature that partakes of the feast is cured of all diseases and poison. All fatigue is removed.

Drink to Good Health

In their final ceremony they will drink from three chalices each to signify a portion of their oath, while it give the leadership an opportunity to make sure this new knight apparent is not under external influences. They will speak their words and receive the mark of the veil.
The liquid prepared for each chalice is a form of holy water augmented with powerful radiant magic.

Greater Restoration - D&D 5th Edition

Basic Rules, pg. 246

Greater Restoration

5-level Abjuration

Casting Time: 1 Action

Range/Area: Touch

Components: V, S, M*

Materials: * - (diamond dust worth at least 100 gp, which the spell consumes)

Duration: Instantaneous

You imbue a creature you touch with positive energy to undo a debilitating effect. You can reduce the target's exhaustion level by one, or end one of the following effects on the target:

One effect that charmed or petrified the target

One curse, including the target's attunement to a cursed magic item

Any reduction to one of the target's ability scores

One effect reducing the target's hit point maximum

Available for: Bard, Cleric, Druid, Artificer

With the gift of these crystal waters I cleanse my mind, body and soul promoting the free and sovereign nature of all.

With the gift of this celestial milk I Armor myself against the foul things of the night.

Mark of the Veil

Each Knight of the Veil receives a Mark of the Veil which acts to show their allegiance. This mark is part arcanr mark and part mystic taboo. Each mark is different and unique to the knight. The mark is scribed on this living skin and the tome of the order at the same time.
In addition to being part ceremony the mark has some very practical applications as well. It is very difficult and may impossible to alter with outside magic. A cleric or paladin of the knights may confirm the bearer is not under influence or an imposter with a quick glance and application of radiant power.