you have to kill the detainer astro toilet in either endless or endless hardmode, I would recommend hardmode because he spawns more frequently in there, I would also recommend to get some people with you because of the new buff in hardmode, also you gotta make sure either you or one of those people actually have beaten 50 waves in regular endless so that endless hardmode can be started, i typed way too much about this
you have to kill the detainer astro toilet in either endless or endless hardmode, I would recommend hardmode because he spawns more frequently in there, I would also recommend to get some people with you because of the new buff in hardmode, also you gotta make sure either you or one of those people actually have beaten 50 waves in regular endless so that endless hardmode can be started, i typed way too much about this