The Harrower does not spawn naturally. You have to call it.
And no, it isn't going to be particularly happy about that.
Before you begin
You need a Hollow Fragment, found through exploration, and a Hollow Cairn built from polished deepslate. Bring your best equipment and choose an open area. This is an endgame encounter, not an experiment to conduct beside your storage room.
Construct the Hollow Cairn
Place 10 polished deepslate blocks in a flat, rectangular ring, with a two-block opening in the middle. Viewed from above, the intended footprint is four blocks long and three blocks wide:
D D D DD . . DD D D D
D = polished deepslate. . = empty space. All blocks are on the same level.
Perform the ritual
Throw a Hollow Fragment into the opening. When the ritual activates, the stone darkens and pale souls surge upward. After a brief silence, the Harrower emerges nearby and immediately turns its attention to the summoner.
The Cairn can be reused, but another Hollow Fragment is required for each summoning.
A final warning
Give yourself room to move. The Harrower is enormous, and its attacks punish anyone who assumes standing still in good armor counts as a strategy. Once the ritual begins, don't expect a polite introduction.