The Face of the Ancient is a special building and is one of two unique decorations released exclusively for the Guild Expeditions. It can be obtained as a reward for a successful Guild Expedition encounter or found in a relic.
The Face of the Ancient provides happiness. When the Face of the Ancient is polished its output of happiness is doubled for 12 hours.
Together with Gate of the Sun God, it was one of the best happiness providing buildings available in the game, providing much more happiness per tile than premium decorations and even premium cultural buildings. However, the game update 1.95 seriously nerfed the happiness value - it now provides only 56% of the original happiness value, thus making it only a bit better than regular decorations and worse than premium ones.