You must summon the undead dragon Durnehviir three times to learn all three words of Soul Tear.
Rather than doing all the boring gameplay content (yes I'm kidding don't worry) we can use CCs instead.
- Go to an open outdoor area - a quick one is "coc whiterun".
- First learn the following shout Summon Durnehviir.
- "player.teachword 020030D4" then "player.teachword 020030D6" then "player.teachword 020030D7".
- Go into your Magic tab, then spend 3 dragon souls to Unlock the full use of Summon Durnehviir.
- If you don't have any dragon souls, use this 3 times first - "player.modav dragonsouls 1".
- Now equip the Summon Durnehviir shout and aim at the ground in front of you.
- Use your shout at full power (hold Z on keyboard), the dragon appears, teaching word 1 of Soul Tear.
- Now wait 1 hr (T on keyboard), then summon him again to learn word 2.
- Now wait 1 hr (T on keyboard), then summon him again to learn word 3.
- The achievement should unlock.
To use CCs (console commands) press ~ on your keyboard, type the command you want, then press enter.
Some good CCs:
*** Spoiler - click to reveal ***"tgm" toggle god mode (infinite health and magicka)
"tcl" toggle no clip (flying)
"sgtm x" speeds up the game, x is how fast (sgtm 10 means 10x speed, sgtm 1 means normal speed etc)