The guardians are actually unrelated to STK spawning, they are just for fun!
STK spawns at a regular time, it's slightly over 8 hours per respawn. In the middle of the main floor, but in a statue state so he won't attack or move but also can't be attacked
At the exact same moment, each of the 4 portals has a knight spawn defending it and a pillar. Clear the pillar to get in, then in the next floor kill the 2 pillars per floor (they will be defending by frost giants). Each of the 2 ice pillars per floor will drop a power orb.
Collect all 8 power orbs, then walk near the STK and he will wake up from his statue state, as will his protectors, the 8 svaltus elites
Beware of traps around STK, if you're killing it solo then it might be helpful to lure him away a bit first. But be aware, he also has a dangerous reanimation skill!
There's directions to where each thing is on the map here
https://zade-mir.com/maps/
Hope that helps!