Well I thought that Urdak IS actually the Father or better - The Fathers body transformed into Urdak, while his consciousness remains in the new Body (Urdak). The Maykrs do need to be replaced after several years since they are not immortal. The Khan Maykr in Doom Eternal should've been replaced as already said, but the Seraphim stole the Fathers Essence or better his consciousness. Since the Father is needed for the replacing of the Khan they couldnt do it anymore and the Khan in Eternal had to stay Khan which was not intended which probably is the reason why he turned bad. theres the Theory that Samuel hayden is the Seraphim, who uploaded himself on the robotic body. (His backround story as a human has basically no proof). Thats the reason why Hayden knows EVERYTHING about the backstorys etc. (Samual and Samur sound similiar too). Hayden then created VEGA and used the Fathers essence as a kind of unconsciousness of VEGA so VEGA doesnt know about it. (Hayden said in DOOM 2016 "I offer you a gift. Take it. it will give you strength" just like the Seraphim said in the Doom Eternal flashback). Hayden also says in Eternal that everything could have been a lot better if it was under different leadership (he means himself). My theory is that the Seraphim wanted to be the new maykr but somebody else was choosen to be. So he stole the Fathers essence and stopped the "ceremony" for the new maykr. he then proceeded to do everything he can to kill the Maykr. Since he himself was less powerfull than the Khan he tried to create something better (The slayer). He then tried to bring VEGA to urdak and the Slayer with him to kill the Khan. That way the Father is again in his body and a new ceremony can occur. Maybe hayden/Seraphim wants to upload himself in that big preserved body to be the new Khan. So that big slimy bastard was probably meant top be the Khan back in the days where the father was still inside urdak (btw i read that Hayden and the Seraphim have the same voice actor.)