DoS Julius Mode is probably the best alt mode in any of the IGAvania games. It's the most complete, with its own story and alternate final boss. It's what all the other alt modes should have been.
Not to mention the feels harkening back to CVIII with interchangeable characters(Belmont>Belnades>Alucard) with a better final boss.
An alternate mode that’s so complete, it could’ve replaced the main one.
I forgot to mention that while DoS and AoS have great boss battles, Death alone in DoS tips it over the edge. The Paranoia boss/ element was cool as hell, as well.