Saturn is the planet of time, limits and work. Pisces is a mutable water sign ruled by Jupiter: dissolution, compassion, faith, endings and everything that has no edges. Jupiter is neutral to Saturn, so the planet is neither welcome nor refused in a sign that tends to blur what Saturn wants to define.
The theme is boundaries in boundless things. Care for the suffering needs institutions. Endings need handling. Faith is asked to be practical. The lesson is that compassion without structure drowns, and that the last sign is where a cycle is closed properly before the next begins.