Does the Rek'Sai legend's ability allow a player to play a card for free after conquering or do they have to pay the cost of the card they keep?

When you use the Rek'Sai (Void Burrower) legend ability to play a card, you must pay the full cost of that card.

According to RiftJudge FAQ #284:

• "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."

Because Rek'Sai's ability does not contain the phrase "ignoring its cost," you are required to pay the card's energy and power costs as normal when you play it.

Note: I could not fully verify this ruling against the retrieved rules.