Chaos Knight vs Mars

How this matchup plays

Landing a big Arena of Blood feels awful against Chaos Knight. His Phantasm illusions are not contained by the wall, so they simply walk out and attack Mars or his allies from behind, making Bulwark's frontal damage reduction useless. God's Rebuke can only knock away one threat at a time, and the rest of his kit is single-target, leaving Mars with no real answer to the swarm. This core interaction is also why they work poorly together: Mars traps enemies where CK's primary damage source, his illusions, can't follow.

In teamfights, the Chaos Knight player can wait for Mars to commit his Arena, then use Phantasm and Reality Rift onto a priority target. The Mars is forced to either abandon his own ultimate or get surrounded and killed. The only way for Mars to handle this is if his own teammates bring overwhelming area-of-effect damage to clear the illusions for him. Without that specific help, Mars can't control engagements the way he's designed to and often becomes a liability.