Stats Scrolls
Defeating the Harrower grants stat scrolls, which increase the maximum total stats a character can gain. The highest-level stat scrolls are +25s, and the lowest are +5s.
Summoning: The Harrower is not a mere spawn that you go battle, he must be summoned. To do so, you must first collect the 6 Champion Skulls, Skull of Greed (Vermin Horde Spawn), Skull of Venom (Arachnid Spawn), Skull of Death (Unholy Terror Spawn), Skull of Pain (Abyss Spawn), Skull of Power (Cold Blood Spawn, Ice Champ), and the Skull of Enlightenment (Forest Lord Spawn) place them on the Star Room altar at Terethan Keep. Once completed, a black moongate will open revealing the Felucca dungeon the Harrower picked for battle. Here is a list of all locations where he may nest:
Covetous - Level 2 in the large central chamber or Level 3 in the Throne Room
Deceit - Level 1 in empty room near the entrance or Level 2
Despise - Level 2, South of Orge Lord Island
Destard - Level 1, either in the main area or in the back near the shrine
Fire Dungeon - Level 1 in the cemetery or Level 2 near the center
Hythloth - Level 3 on the blood pentagram or Level 4 near the center
Shame - Level 3 in front of the West or East Mage Towers
Terathan Keep Underground - In the swamp where the Nightmare spawns
Wrong - Level 2 in the mess hall