A mainframe is just a very large server, with lots of reliability features (RAID-like memory, fault detection and mitigation, redundant components, etc) and lots of intelligent peripherals that offload work from the CPU so that it can spend as much time as possible running application code (and don't waste time handling interrupts, assembling network packets, dealing with IO, etc). A lot of these offload functions are baked into the ISA, making it a VERY CISC machine.

I believe Unisys still makes x86-based mainframes running MCP.