Reach Choir in Notre-Dame
On the normal story route, enter Notre-Dame through the Artist painting, finish the three organ scores and progress through the local Death encounter. Continue along the upper church passages to Choir.
Look for a yellow glowing doorway beside a large gap. Approach it to trigger Leon’s dialogue and receive Double Jump.
Test the new ability
Jump, then press your Jump input again while airborne. Use the second jump to cross the gap and reach the platform above the spike pit.
Soul Harvest is a separate spell. Death awards The Shadow / Soul Harvest; the later Leon conversation unlocks Double Jump. You do not need to treat Soul Harvest as a jump spell or equip it to perform the second jump.
If the dialogue has not triggered
Check that you have progressed the Notre-Dame story and reached the correct doorway in Choir. The unlock is much later than the opening Paris streets; revisiting early ledges will not reveal it there.