Olgierd von Everec was introduced in trailers as main antagonist od Witcher 3: Hearts of stone, but it was a misinformation campaign to hide the true main antagonist. Olgierd is an arrogant Redanian noble, who wanted to marry Iris but their parents didn't allow for this due to his bandit lifestyle and poverty. They arranged marriage with an exotic prince instead.
Olgierd made a deal with Gaunter O'Dimm to become immortal, so he could restore his fortune and marry Iris. To do this he sacrifaced his brother and magically changed to prince into a toad monster and imprisoned him in sewers. He succeeded but began to lose empathy and other emotions. Because Iris isn't immortal and age he imprisoned her in a different magial dimension to keep her alive.
He has to fulfill his part of deal with Gaunter so he asks different people to make impossible tasks for him. Geralt of Rivia is one of them.