Nimrod the Scarlet Sentinel was a lightning rod monster created from a stone fist statue by Lord Zedd. She had two effeminate looking male assistants named AC and DC. Nimrod was extremely powerful and the added strength of AC and DC made all three of them even more deadly. They were able to topple the Thunder Megazord, leaving the Rangers temporarily defeated. While normally the Thunder Megazord would have gotten up and finished them off with the Thunder Sabre, like against Robogoat and Stag Beetle, they had run out of energy due to the Command Center shutting down (as seen later in The Wedding). The Thunderzords were unable to continue battle, the Command Center being shut down caused a deficiency of power. After Tommy gained the powers of the White Ranger, he was able to take on the monsters. Nimrod and her cohorts AC and DC were the first to face the new White Tigerzord and the first to be defeated by the Mega Tigerzord.
In the Power Rangers: The Movie game for the Sega Genesis, the monsters were referred to as S. Sentinel Ring, S. Sentinel Ear, and S. Sentinel Neck, a reference to the Dairanger names of the Three Ladies: "Lady Ring", "Lady Earring", and "Lady Necklace".
"AC" and "DC" mean "alternative current" and "direct current" (this is also the origin of the name of the popular rock band AC/DC), and the three of them have electrical powers.
It is unclear whether AD and DC ceased to exist or retreated when the Scarlet Sentinel was destroyed.
Nimrod the Scarlet Sentinel is probably named after the Marvel Comics character Nimrod, a Sentinel from an alternate future, which is also red.