My pcs have killed all the constructs and the elemental outside his door. One of them dueled with Cerssee and lost so she is with him as well.
The party is planning on resting before they enter the final door. So what would Yrax do? He knows they're in his castle and that they are there to try and kill him. He wouldn't just sit there and wait would he?
He is the kind of arrogant leader that will gladly use his underlings as cannon fodder. But no warlord would allow the enemy to enter hit home, kill all minions and then retreat to rest before facing him.
He has a fine perception score. He can hear them battling his guard and will likely start preparing for a fight. With a perception of +25, he can easily eavesdrop Om the players while preparing his ambush.
If they want to retreat, I'd have him follow them. No way they are getting away with killing all hus minions. He's a dragon, he's snart enoygh to know they are woulnerable and use it against them.
Yrax has an entire army waiting right outside the palace.
Also, there's silent alarm spells all over the palace, so odds are very good Yrax knows about where the PCs are at any time.
If the party has made any allies at all, you should make clear that retreating from the palace could have disastrous results, up to and including resetting the dungeon =P
If they are barricading the doors and camping in the palace?
Yeah, Yrax needs to say hello to them with extreme prejudice while they're camping. Sleeping in a hostile dragon's kitchen/library/lab/antechamber/throne room is an act of incredible hubris.
Since Cerssee won she has reported back to him so he knows for sure they are there. I was thinking that he would do something for sure. Seeking them out does seam like a good option. Filling his place with soldiers seams another.