So, you crack a Rust Phoenix egg before it is ready to hatch and you get a little smear of long lasting residue that (provided you know a variant of Teleport, Planar Step, or Dimension Door pretty well) you can BLOOP right over there to where the egg was broken, or pretty close. A paradimensional scramble right there. Easy to commodify. Very cheap to produce, highly useful, and pretty delicate and short-lived. Because If you don’t break the egg, then the little bird’s accelerated embryonic development completes and it hatches at full size, irritated, and causes weird degeneration of non-magical metals. Rust. Every metal thing within about a 100’ radius of a properly incubated Rust Phoenix egg gets rusty, pocked, and pitted.
This Beast Wizard, he captured a male and a harem of females and he pumps out a fortune, selling the eggs (for which there is a long waiting list) for a king’s ransom to teleporting-interested groups and also for warfare.
The males are generally very striking in terms of appearance: jagged, sharp, crumbly, prickly. An amazing ventral plume that is dangerously sharp. A fine sediment of rust precipitates from the air around them and settles on things. Also, the males have a warbling and haunting song that causes metal objects nearby to degenerate and shatter - magical items are more or less resistant but common metal ones can actually cause damage to bearers by forceful explosion into fragments
Rust Phoenix DCC Stats (I don’t know, just regular bird stats let’s not dwell on minutiae, eh?)
Rust Phoenix Dungeon World stats (maybe more on that later, but a motive/driver definitely to escape captivity, nest in secure locations, revenge upon captors I mean no need for a noble beast to be a good one)
Related spells (Transition Egg, Magnetic Cascade, Singularity Song)
Related items (Rust Phoenix Egg, Rust Phoenix Down, Feathers)
Caught in a Faraday cage, warbling a sinister tune, a legendary beast become a machine for the wealth of a greedy merchant