Verified for patch 16.8.1

Eternal Pact

Combat start: Form a pact with the highest Health allied champion, if they die, gain 10 Mana regen and 40% Ability Power.

When the ally uses their ability, gain 15 Mana. Every 6 seconds, shield your ally for 200% of the holder's Ability Power.

Source

Artifact

Effects

AP: 40
APToGrant: 40
ManaRegen: 1
ManaRegenToGrant: 10
ManaToGrant: 15
PercentAPShield: 2
ShieldCadence: 6
ShieldDuration: 10

Eternal Pact enables

Shield On the Menu

Grants temporary HP that absorbs damage before the unit's real Health is touched.
Mana On the Menu

The unit-level resource that gates ability casts. Mana-gen items, on-cast refunds, and starting-mana augments all key off this mechanic.

Eternal Pact scales with

Ability Power On the Menu

Scales with Ability Power. Damage and effect magnitudes that read "(scales with AP)" target this mechanic.