SUPERBOY 184 (April 1972)--2nd story, "One Legionnaire Must Go" by Bates, Anderson and Cockrum
SUPERBOY 188 (July 1972)--1st story, "Super-Savage--Master of Metropolis" by Dorfman, B. Brown and Anderson; 2nd story, "Curse of the Blood Crystals," by Bates, Cockrum and Anderson
For the Legion back-up series in SUPERBOY 184, Murphy Anderson is the new penciller with his assistant, Dave Cockrum doing the inks.
However, Murphy knew that Dave had a passion for the Legion, so the next assignment (issue 188), Anderson invited Cockrum to handle the pencils and Murph would do the inks.
This tale begins at the end of a Superboy encounter with Mordru in the 20th century. The Kid from Krypton defeats the Sorcerer's World Reject by using his one weakness against him, burying Mordru under the earth. The Teen of Steel takes the Blood Crystals, that the Merciless Mage created to make the Smallville Samaritan an outcast, and throws them into outer space.
Ten centuries in the future on board a Legion exploratory space station, Reep Daggle (also known as Chameleon Boy) happens to find the Blood Crystals in space and comes under their spell. Posing as Ultra Boy, he takes a Time Cube back to the 20th century.
When Brainiac 5 and Lightning Lad are alerted by the real Ultra Boy to Cham's subterfuge, they use another Time Cube to visit the Caped Kryptonian. They materialize in 1950s Smallville and Superboy explains that Reep was under the spell of Mordru's Blood Crystals, but he's using a Remedy Roy to cure Chameleon Boy. Querl and Garth are satisfied with this explanation and head back to the future.
However, "Superboy" is really Reep, still under the influence of the Blood Crystals and the "Chameleon Boy" is actually Clark, who Cham had trapped by taking the form of an elephant and blowing green K. at him with his trunk. With the green stuff on his chest, Kal-El is slowly dying.
But the Red and Blue Blur suctions the Kryptonite off his chest and blows it away then buries the Durlan underground, reasoning that the same trick that neutralizes Mordru would also work against his magic.
Note: Given that Mordru continues his vendetta against Superboy in the 20th century, I think this strengthens the case for Mordru being a 20th century villain as much as he is a 30th century one.
The Superboy cover story in this issue continues the imaginary story of Karkan the Mighty, as the Lord of the Jungle is persuaded by his new girl friend, Toni, to go to the big city. He doesn't like what he finds there and elects to return to his savage land.