Heidi Klum Debuts Halloween Costume ... Her Bloody Severed Hand

Heidi Klum's latest, highly anticipated Halloween costume is as familiar to her as the back of her own hand because ... well ... it's her own left hand.

The supermodel transformed into an 8-foot version of her bloody, severed left hand ... complete with her iconic thumb ring and her wedding ring to husband Tom Kaulitz.

Trading red bottoms for red fingernails as feet ... the former "America's Got Talent" judge showed up to her own Halloween party -- its 25th anniversary -- at NYC's Moxy Hotel in the gory and graphic costume.

Apparently the inspiration for this year's costume came to her exactly a year ago ... when she was being fitted up by Hollywood makeup artist Mike Marino to portray Medusa.

She told the New York Times ... "I remember sitting in the chair, and I said to Mike, 'Do you want to know already my idea for next year?' And he's like, 'Please, no, I'm focusing on getting Medusa squared away, I can't handle this right now.'"

Mike later scanned her hand for reference -- recreating every fine detail of her phalanges.