Say a preist casts 2 heals while fade is activated and generates X amount of threat, is the same amount of threat over time generated if the priest casts the same exact heals and THEN fades?

Fade is a temporary threat reduction for a set period of time (6 or 8 seconds; whatever the tooltip says). It lowers your threat by a static amount (depending on rank). You still generate hate while faded, and when it wears off, the threat you lost from fading gets re-added to your current threat. It makes no difference for "total threat generated" over time.

For rank 6 (lvl60) it's -820 Total Threat for the duration, then back to normal like you never used it like Thrillho explained.

820 Threat =
(0/5 silent resolve): 1640 hp healed.
(5/5 silent resolve): 1886 hp healed.
(5/5 silent resolve + subtelety on cloak): 1919 hp healed.

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