After awakening, and when exchanging my skills the Angel of Death skill did not show any skills to-be-deleted, so I assumed that Wind Riding (lv 83) will be used to replace an other skill later. After getting Angel of Death I could use both Wind Riding and that skill at the same time. After re-log/restarting, Wind Riding was disabled/unable to use.
Focus Death skill was disabled after awakening, however, focus chance was still there and could be used. After log-in, the skill was disabled.
I talked with an adventurer, and he noticed similar behaviour with skills :Mirage and Exciting Adventure.
(PS. If I did not make the wrong assumption, I would keep Wind Riding for some more levels :()