What presidential power did abraham lincoln use when he introduced the emancipation proclimation (ended slavery)?

Lincoln Issues the Emancipation Proclamation. One month later, after the qualified Union victory in the Battle of Antietam, Lincoln issued a preliminary proclamation warning that in all states still in rebellion on January 1, 1863, he would declare their slaves “then, thenceforward, and forever free.”

I believe this states that he used he would declare their slaves “then, thenceforward, and forever free.” As a promise. A commitment. Something he would keep and fight for.