The cleanup after Graves's ability may kill your last defender,^323.4 but the ongoing combat prevents control of the battlefield from changing.^190.4.b You therefore still control the battlefield, and the facedown card stays there.^811.1.b^323.6.a The facedown unit can still be played.^811.6

Example

Graves attacks alone, and his ability kills every defending unit. At your next legal opportunity, you play Evelynn, Entrancing from face down at that battlefield. Graves deals no combat damage, and Evelynn's 2 deals only 2 damage to the 4 Graves, so neither unit has lethal damage.^142.4.b^467.2.b^467.2.c^467.2.d Because Evelynn remains as a defender, Graves is recalled.^468.1.a.2 Evelynn's controller retains control, so Graves's controller does not conquer the battlefield.^468.3.d^468.5^471.1

From Riftbound FAQ by Christian “Near” Ivicevic, licensed CC BY-SA 4.0. Reviewed against Core Rules 1.5. Unofficial; not from Riot Games.