Some TMNT stuff was always meant for grownups.
Lord Simultaneous is the master of time and Renet and first appears in Eastman and Laird's Teenage Mutant Ninja Turtles volume 1 issue #8: Team Up with Cerebus. The same individual character also exists in the IDW continuity, first appearing there in the Teenage Mutant Ninja Turtles 2014 Annual.
A methodical and dedicated Time Lord, committed to keeping the time stream of Earth clean and unpolluted. He has trained several apprentices over the years and had found life with his latest apprentice, Renet, to be by far the most difficult. Prior to taking on Renet, Savanti Romero was his apprentice, who ultimately turned on him and tried to "edit" him out, forcing him and the 79th Level to banish him to 1406 A.D. He did, however, have high hopes for Renet, since he saw something of his own youthful rebellion in her. Although Renet in her first misstep in which she met the Turtles for the first time and in a chain of events, Savanti Romero was freed from his exile in the Middle Ages, and was harshly punished, Renet managed over time, to become a true time guardian and possible successor.
In issue 8 of Teenage Mutant Ninja Turtles, Renet sneaks into Lord Simultaneous' personal chambers to steal his Time Scepter so she could escape her 'boring' life. Against his wishes, Renet once time-traveled off with Lord Simultaneous’ Time Scepter to modern-day New York. She teleported to New York in the year 1986. Simultaneous was angry, and followed her. He followed her throughout dimensions trying to regain his scepter. Fleeing Simultaneous further, she and the Turtles went to 1406, where they teamed with Cerebus to get the Sceptre back from Savanti Romero, who was bent on stealing it for himself. Simultaneous appeared and cast Savanti into prehistoric times