Well, that sure looks like Swamp Thing on the cover of Phantom Stranger 14 (July/Aug 71), doesn't it? But nope, it's not. And despite the image of the Stranger, the Adams cover actually pertains to the Dr. Thirteen story in the book.
Len Wein scripts both of the stories in this issue. The Phantom Stranger tale, with art by Aparo, opens in quite a shocking fashion, with the character being invoked by Broderick Rune, and taken prisoner. Rune then promptly dies himself, from a heart attack, and his faithful servant oversees an operation that replaces Rune's heart with that of the Stranger.
The Phantom Stranger visits Rune to warn him about this unwise course of action in a dream, and then continues to haunt the man.
It's all very creepy, and quite different from the stories that have run in the book so far. The Phantom Stranger returns by the end of the tale, none the worse for wear, and Rune dies again. In the autopsy, it is discovered that he has no heart at all. Oooooooh, spooky!
Although we do not see Rune again, him being dead and all, he will later on be revealed to have been involved with the Dark Circle.
Tony DeZuniga does the art on the cover story, in which Dr. Thirteen and his wife Marie head to the Louisiana bayou to investigate rumours of a swamp creature, which has been carrying people off. Terry heads out alone to check this out, and becomes the swamp creature's next victim.
When he doesn't return, Marie gets the sheriff to lead her out, and they find a weird domed village deep in the bayou. The swamp creature is the leader of the dome, just a man in a costume, but a scientist with a weapon that accelerates swamp growth. He uses it to take out the sheriff.
The guy has been assembling a society under his control, which will survive the outside world, which he feels is heading towards destruction. He orders a hypnotized Dr. Thirteen to kill Marie, but Terry breaks free of his programming and fights back. The dome gets damaged, and the accelerator causes the plants to go wild and crush it completely, killing the guy.
My brain is telling me that this domed place does return in an issue of Swamp Thing, but I might be wrong about that, combining a different location with this one.