Mark Grayson’s arms were blown off in his brutal fight with Conquest, and by the time Invincible season 4 opens, he’s using robotic replacements while he recovers.

What happened

Mark takes massive damage during the Conquest battle, which leaves him gravely injured and leads to the robotic limbs seen afterward. The comic continuity also shows that the fight with Conquest is the turning point that leaves him in very bad shape, with recovery taking months.

Why he has robot arms

The robot arms are basically a medical and practical fix after the injury, not a permanent power upgrade. They let him function while his body heals, even though they clearly aren’t as natural as his original Viltrumite limbs.

In plain terms

Question| Answer
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What happened to his arms?| They were destroyed in the Conquest fight 51.
Are the robot arms permanent?| They’re a recovery solution, not a new normal 5.
Is this from the comic or show?| Both the comic context and the season 4 setup point to the same injury 15.

Spoiler-safe takeaway

So the short version is: Invincible’s arms didn’t “randomly change” — he got wrecked in battle, and the robot arms are there because he’s badly injured and healing.