Willowdusk, Essence Seer is a mythic Legendary Creature — Dryad Druid card from Magic Online Promos (PRM), collector number 90288.
{1}, {T}: Choose another target creature. Put a number of +1/+1 counters on it equal to the amount of life you gained this turn or the amount of life you lost this turn, whichever is greater. Activate only as a sorcery.