The killing of Bishop Henry by Lalli occurred on the frozen surface of Lake Köyliö and is a significant event in Finnish folklore and history. Bishop Henry, an English cleric, had been active in disseminating Christianity in Finland since around 1155, encountering resistance from local pagan communities. Lalli, a local peasant, reportedly killed Henry during a confrontation that stemmed from resentment towards the bishop's efforts to impose foreign beliefs upon the native people. According to legend, Lalli was provoked by the bishop's attempts to guide him towards Christianity, leading to a moment of violent rebellion against the changing landscape of religious belief in the region.

This event is not just an isolated act of violence; it highlights the resistance faced by missionaries and clergymen during the Christianization of Finland. The tale of Lalli and Henry survived through oral traditions and later written texts, symbolizing the clash of the old pagan ways with the new Christian beliefs being brought by outsiders. The conflict illustrated the broader societal changes occurring in Europe during this time, where many regions were experiencing similar transitions under the pressures of new ideologies.

In the aftermath of Henry's death, his story morphed into a martyr narrative within the Catholic tradition, elevating him to a saintly status. This transformation would have profound implications for the establishment of Christianity in Finland, as the legend of his martyrdom laid groundwork for the future consolidation of the Church's influence in the region, thus shaping Finnish Christian identity for centuries to come.