Sentinel's Mark
Enchantment — Aura
Flash
Enchant creature
Enchanted creature gets +1/+2 and has vigilance.
Addendum — When Sentinel's Mark enters the battlefield, if you cast it during your main phase, enchanted creature gains lifelink until end of turn.
Illustrated by
Ravnica Allegiance#20UncommonEnglishLegality
Rulings
Jan 25, 2019
If Sentinel’s Mark enters the battlefield without being cast, the addendum ability won’t trigger, even if it’s your main phase.
Jan 25, 2019
Gaining vigilance any time after the moment you choose to attack with a creature won’t cause it to become untapped, and losing vigilance after that time won’t cause it to become tapped.
Jan 25, 2019
Multiple instances of lifelink on the same creature are redundant.