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.