WARNING: The following contains spoilers for Young Justice: Phantoms, streaming now on HBO Max.
As Young Justice: Phantoms has progressed on Mars, there has been a nervy sense of tension. A lot of it is due to M'comm (Miss Martian's terrorist brother) planting a bomb in the royal chambers to make his people pay for their oppression, but there's also been a feeling like one of the traveling contingent from Earth may suffer too. Well, in the fourth episode, "Involuntary," the show wades into the latter, making a stunning play by killing its most powerful hero.
The brutal death occurs when Prince J'emm makes his speech about unifying the planet and breaking down the caste system. He wants Red, Green and White Martians to live as one, but as it happens, a shadowy figure slips into his chambers below and activates the bomb. Conner senses something's off, uses his x-ray vision and opens up a hole so he, M'gann and Gar can jump down.
Sadly, time's running out and they can't take it to the surface, which results in Superboy bashing another hole in the ground to drop it into the lava pit below. He has to do it alone as he doesn't know what effect it'll have on M'gann or Beast Boy as his microscopic sight confirms how unpredictable the chemical mixture is.
The problem is, as Connor jumps into the lava to detonate the weapon, the toxic substance seeps out, poisoning him as it's burned away. The bomb then explodes, killing Superboy in what is one of the series' most stunning sacrifices. The sad thing is, the atmosphere is so lethal down there, Martians -- even Martian Manhunter -- can't enter, especially as fumes from the bomb are still lingering. The heat will also kill them so it's basically nature cordoning off a zone.
Another big reveal comes when Superman tries to fly down but can't. It's confirmed Kryptonite was in the mixture which means it catered for him or Conner in a truly methodical move. As a result, no one can search for the body, with M'gann crushed as his mind link has disappeared. All that's left is the ashy outline of the hero, which has her in the post-credits crying in the chamber above. Some folks are hoping the Legion of Super-Heroes, hidden on Mars in secret, did intervene and pluck Conner out, but the outline more or less confirms Superboy bit the dust as the magma subsides.
It's a gruesome, cruel fate to see an untouchable tomb like this, which also has fans wondering if other Justice Leaguers can come over to find Conner so they can have a proper burial and grave.
That said, seeing M'gann mourning her beloved is akin to when Wally sacrificed himself in the past as well. He left Artemis behind and now, Conner's sacrifice is sure to have Aqualad and Co. equally distraught as they try to help his soulmate heal.
To see Superboy's sacrifice, the first four episodes of Young Justice: Phantoms are streaming on HBO Max.