Historical Rules · 5 October 2018
Rule 702.102d
Official source text from the 5 October 2018 Rules release
As an Aura spell with bestow begins resolving, if its target is illegal, the effect making it an Aura spell ends. It continues resolving as a creature spell and will be put onto the battlefield under the control of the spell’s controller. This is an exception to rule 608.3a.