how can i make mobs apper on syrten phaze?

make a GLOBAL_WORLD variable, make a procedure with a "when player wakes up" preset, and make it change the variable by adding 1. then make it check what number the variable is at. depending on what the number is, from 1 to the maximum phase of your choosing, make it progressively more difficult by making more mobs spawn the higher the variable is. let the variable run infinitely, and when it reaches the final phase, make it check if the variable is above what it was back in the previous phase.

you could also make it not change the variable if it's at the final phase, which would reduce lag.

If you stages that aren't dependent on the days, make a logic gamerule for each stage, and set the corresponding gamerule to true (and the previous on to false each stage if you want,) in however you want it to be triggered (Phases of the moon, etc). Then on spawning conditions of the monsters you want to spawn, check to see if the gamerule for the phase(s) you want it to spawn in are true. But this EXTREAMLY painful to do if you want the stages to change increase each day/night.