"Just Say Uncle" from Saga of the Swamp Thing #29 (DC, 1984) by Alan Moore, Stephen Bissette and John Totleben
My first issue of Swamp Thing was one where he fought a crystal man. My second featured this revelation about Matt Cable and Anton Arcane. Creepiest. Thing. EVER. I was then scared of this book for years and never picked up another issue until I was in college.