Sirus is a boss in-game. You need to kill the 4 lesser Elderslayer first before able to spawn the portals to Sirus. Or you just buy the Sirus map frag from other player
It is a dps check, some mind game on escaping a maze, or a corridor , or the Storm that kill you way faster than the boss. The last phase is where is Wally.
Since 3.17.0 the boss fight has fixed area level so that if you can't do it, find a boss carry