The fourth option of the Bestow Curse spell (PHB, p. 218; emphasis mine) states:
[…] While the target is cursed, your attacks and spells deal an extra 1d8 necrotic damage to the target.
The Life Transference spell (XGE, p. 160; emphasis mine) states:
[…] You sacrifice some of your health to mend another creature's injuries. You take 4d8 necrotic damage, which can't be reduced in any way, and one creature of your choice that you can see within range regains a number of hit points equal to twice the necrotic damage you take.
If I cast the Bestow Curse spell on myself, choosing the fourth curse, and then cast Life Transference, will I heal the target of Life Transference for an extra 1d8?
Best Answer
Yes, this works.
You have made the correct observations. Bestow curse causes your attacks and spells to deal an extra 1d8 necrotic damage. If you are the target of that curse, and you target yourself with a damaging spell, naturally, you take the extra 1d8 necrotic damage from the curse. Then, with life transference, because of the curse, the "necrotic damage you take" is 5d8 instead of 4d8, so that extra 1d8 gets counted toward the healing caused by life transference. The crucial point here is that the curse causes extra necrotic damage specifically. If the extra damage was any other type, it would not count toward the healing because only necrotic damage counts.
Finally, as a side note, you probably cannot use life transference to heal yourself.