Spoiler - how it was happened - could be found in comment at oookMoment of the death:
1) something summons Bile Demon. I can't udnerstand what and tele away
2) after I teleported to the new location - I open the door and I've met some other mobs; right there again something summoned Bile Demon who finished me
I've checked with my high lvl char - it was Strygalldwir ( http://muuttuja.org/tomenet/monsters/monster.php?651 ). He is invisible, BUT I got see_inv from amulet (Platinum Amulet the Moon). Why I didn't see him? I wasn't blind (I see other mobs).. He was at too big distance? But how he managed to summon then if he was out of LOS?