au! => in which loki uses his magic (sceptre) to save tony’s from his poisonous palladium core but realises that it is only a temporary solution. in fact, the poison has transferred from the palladium core to loki’s magic. hence, everytime loki uses his magic, he is poisoning tony too.
the thing that is keeping you alive is also killing you.
does this make sense? i’m just substituting the new element with loki’s magic. but there’s a catch, the more loki uses his power/magic, the more poisoned tony becomes. tony is on the verge of death but he doesn’t want to deny loki of his birthright (loki is the god of mischief), hence keeping it a secret. i hope this doesn’t sound too stupid or confusing.