The field never stopped raising its own dead — The Gleaner is what happens when enough of them get bound into one wrapped shape. It reaps in a wide arc at range; at the halfway point of the fight it comes apart into two lesser Chaff Wraiths and keeps fighting, permanently a point worse for the split; and with its last breath it lets out one lungful of grave-rot.
A scythe-wide bleed around itself — 2d6+3 slashing damage to anyone within 3 meters, and a failed Fortitude save (DC 15) costs a point of Constitution for four rounds.
It comes apart at the bindings: two Chaff Wraiths peel off and join the fight, and what's left of the original hits a permanent point worse for having given up two of itself. The module's first split — every other boss stays one creature the whole fight.
One last lungful of grave-rot: everyone within 7 meters who fails a Fortitude save (DC 16) fights sickened, at −2 to hit and −2 on all saves, for four rounds.
Revised CR7 → CR10 after checking The Outer Fields' ambient table: real content there runs up to CR8 (Hive Guard), so a CR7 boss was actually weaker than the area's own giant ants. CR10 clears that with room.
114 hp split into three bodies mid-fight (the original plus two CR2 Chaff Wraiths) works out to noticeably more total hit points than a single boss would normally carry — that's the point. The Gleaning is meant to feel like the fight gets harder before it gets easier, not like free extra loot; the wraiths were stripped of boss-tier loot flags for exactly that reason. Con 16 is unusually high for an undead build, kept specifically so the expected-hp formula produces a body worth splitting in half without the math feeling forced.