Regna, the Redeemer

The second ability represented by the "partner with [name]" keyword modifies the rules for deck construction in the Commander variant and has no function outside of that variant. If a legendary creature card with "partner with [name]" is designated as your commander, the named legendary creature card can also be designated as your commander. For more information on the Commander variant, please visit Wizards.com/Commander.

If your team didn't gain life during the turn before the end step begins, Regna's last ability won't trigger at all. Gaining life during the end step won't cause the ability to trigger.

Note that the target player searches their library (which may be affected by effects such as that of Stranglehold) and that the card they find is revealed, even though these words aren't included in the ability's reminder text.