The Necromancy wizard's Grim Harvest feature allows the necromancer to heal themselves an amount of HP equal to 3 times the level of the necromancy spell used to kill a creature, once per turn.
Instantaneous spells would just heal once per casting. Spells with a longer duration could kill one creature per turn for many turns for repeated healing.
Assuming at least one creature dies each turn, what is the maximum amount of healing a necromancer can get from the Grim Harvest feature from a single casting of a necromancy spell?
Best Answer
Necromancy spell: upcast, extended vampiric touch: 540
Ok, breaking the problem down we care about three things: how many turns a spell can deal damage to a seperate creature, the level the spell is when cast, and whether the spell is a necromancy spell. The last is only a bonus of 50% and we'll asume we have a character with 9th level slots so the spell level isn't a problem either. So now we're looking for a spell which can deal damage to a seperate creature with the longest duration.
Enter vampiric touch, which lets us make one attack per turn for a minute. We can take a few levels in sorcerer for Extended Spell, letting us up that to 2. Assuming a creature dies on each attack makes Grim Harvest restore: $$20\times9\times3=540$$
Mordenkainen’s Faithful Hound: 172 800
While not a necromancy spell so we only regain 2 times spell level each turn, it has a duration of 8 hours, which when we use Extended spell gives us a whopping 9600 turns. Assuming a limitless supply of hostile goons and no misses, Grim Harvest regains us: $$9600\times9\times2=172\,800$$
Do note that we can upcast the spell to 9th level by using a 9th level slot even though the spell gains no benefit from it (other than regaining us more life).