Black Soul Gems

Alternatively, download the mod Shade of the Revenant Altar by GrumblingVomit and MasssiveJuice.

Requires OpenMW 0. 51

This mod takes inspiration from Oblivion Black soul gems, you can transform Grand Soul Gems to Black Soul Gems.

You can only do this on the  Shade of the Revenant, which happens every 8 days, starting with day 8. Check the "Day X" on your rest menu. If the number is a multiple of 8, it is the Shade of the Revenant.

Walk up to the alter, and place grand soul gems in the alter or on top of it(no stacks on top) and cast "soul trap" on it (on the right day). It will transform 1 soul gem per cast. Make sure you have the "soul trap" spell, not a custom or other spell.

Now that you have the black soul gems, you can cast soul trap on most unique NPCs in the vanilla GOTY, including people like Fargoth and Gaenor.

The NPCs you can trap include all NPCs in the base game and expansions, as well as Tamriel Rebuilt(as of this writing), and any NPCs added by "Beautiful Cities of Morrowind"

You can use normal spells, enchantments, or weapons to cast soul trap on people.

Black soul gems can only be used for black souls ( human souls), not animals or daedra. Soul values default to 400 flat, but this can be changed in the settings to any number or based on level.

If you have OAAB_Data installed, the black soul gem from that will be used instead.

There is a ESP included, Black Soul Gems - Message Fix

This fixes the GMST that normally says "Your have trapped a soul". If you have the patch for purists or some other fan patch, it's not needed.

If you don't use OpenMW, you can use Seph's NPC Soul Trapping, likely a much better mod here