The reason you were not aggroed is this dragon, like some other mobs that verant didn't want exploited, are "perma-rooted". This means, even when aggroed, they can not move. Verant did this so people couldn't run him into a wall or something to exploit. though you can still push him with melee *shrug*. So that you can't sit back and nuke him and use trueshot; he, unfortunately, summons T_T.
To answer your question, he has a great deal more hp than nagafen and is 3x harder than Phara Dar.
-He has 160k plus hp (hard to measure because he regens so fast)
-500 plus a tick regen when aggroed and near instant unaggroed regen.
-quads for 700 with close to 1 second dly between rounds.
-AE mana seive dot like wandering mind only faster called velium shards, along with this dot ae hits for 300 and is unresistable.
-100% magic resistant. very high to other resists.
GL with this punk. BTW the sleeper is a mob that's bugged at 0hp and cannot be killed. it does not melee except a rare reposite that quads for 4dmg~. you want to talk about Kerafyrm who spawns after all 4 warders die and goes on a rampage anhillating every person and npc in the zone with deathtouches and has a zonewide aggroe. meaning you have to wait for him to depop because zonein = instant dt.
now that i've ranted enough i'll leave you with one question. Avatar of war + Yelinak + vulak Vs. kerafyrm, no dt that is. that'd be fun