For Rek'Sai's legend ability, do you have to pay the full cost of the unit? Also, if Candlelit Sanctum's battlefield effect triggers at the same time as Rek'Sai's legend ability, which resolves first?

Ruling: Yes, you must pay the full cost of the unit (or non-unit card) when using Rek'Sai's legend ability. Only effects that explicitly say "ignoring [their/its] cost" allow you to play cards for free, and even then only the base cost is ignored.

Sequence: (for Candlelit Sanctum interaction)

• When both Candlelit Sanctum's battlefield effect and Rek'Sai's legend ability trigger from the same event, you control both triggers
• You may order them as you please since you control both effects

Nuances:

• Rek'Sai's ability works with non-unit cards as well, not just units
• You still pay the full cost for those cards unless an effect specifically says to ignore cost