Worship of the old gods had been outlawed by three generations of Northcrests. The current Elias Northcrest went as far to destroy places of worship of the old gods to promote progress. Currently the Trickster has become nothing but a memory, a curse or a swear to used when one is annoyed at another. One such is example is 'Trickster help me'.
Through outlawing, propaganda, institutionalization and preaching the way forward through industrial and technological advancements the people lost focus and stopped worshipping the Trickster. The word passed into old language with little emphasis on the modern. For many, the old god began to pass into legend and superstition.
Although small packets of die-hard believers still practice quietly out of sight (perhaps the Marsh-dwellers are remnants of the faithful), the populous has to larger degree given up on the old gods helping them and turned to the Baron and his promise of brighter and better times. When Garrett is trapped in the The Keep, trying to get the treasure from the Great Safe, he thinks to himself how it better be the Trickster's gold teeth in a jar in the safe, to make it worth robbing it.
Some refer to Moira Asylum as The Trickster's Playground because the madness that exists there.
Crows and rats are considered the Trickster's children.
The Trickster was behind the event where the legendary Sneak Thief (old Garrett) lost his eye. Ultimately the Sneak Thief then defeated The Trickster in return. The eye was later replaced with a Mechanical Eye made by the Hammerites.
The Trickster was the main antagonist and villain in Thief: The Dark Project/Thief Gold. He is mentioned quite a bit in Thief II and Thief III, as the Pagan religion was given more backstory.