Justice League Unlimited Season 2 Episode 16

More Defending
The parallels between this episode and Marvel Comic's Defenders continue - Grundy calls Hawkgirl "Birdnose" (which is what he referred to his Defender ally Nighthawk), Aquaman wields a mystic trident that looks a bit like that wielded by the Son of Satan, and you could probably argue that Wonder Woman = Valkyrie and Superman = Silver Surfer (cosmic powered refugee from a distant planet). There are several similarities between this episode and Avengers 115-118 and Defenders 8-11 when the two teams squared off against each other before teaming up and traveling to another dimension and fighting Dormammu's faceless gigantic minions.

Grundy's Sacrifice
The heroic death of Grundy parallels that in the comic books seen in Starman v. 2 #32-34 or thereabouts. It is there that for the first time we get an explanation for Grundy's occasional mood swings and heroic acts - that when Grundy dies and comes back it's with a different aspect of Cyrus Gold's personality, and that sometimes the buried "good" side comes to the fore.

Icthulhu is Doomed!
Although the Elder God shown here certainly looks like his artwork in the Lovecraft books and related source material, he also looks a lot like the old Superfriends' Legion of Doom HQ.

Ugly Sea Monsters
When Aquaman goes to defend Atlantis we see the demonic creatures emerge from the vortex. The first creature leading looks like a shark with webbed-hands. In a Superfriends episode, Aquaman was transformed into a sea monster by a broken cannister of radiation. The sea monster was a shark with webbed-hands. The other superfriends had to shrink themselves down and used the supermobile to enter the Aquaman creature to drop the antidote onto his brain (similar to the movie Fantastic Voyage).

Ichthulhu:
The whole concept here echoes the writings of H.P. Lovecraft and his creations of Cthulhu and the Elder Gods.
There are many other Lovecraft mythos monsters making cameos; the heroes fight a Gug (the thing with four arms joining into pairs at the elbow), a Servitor of the Outer Gods appears (the frog thing with the big tentacle), and the Screaming Men (the headless gray dudes with mouths in their hands) are copies of Y'Golonac.