If you're already in another dungeon when you would venture into the Undercity, you will instead progress to the next room in your current dungeon rather than enter the Undercity as an additional dungeon or switch altogether
The exception is auras that specifically enchant a permanent you control. For example, Cartouche of Knowledge has enchant creature you control. If you take control of a creature with this Cartouche on it, it falls off since your opponent still controls the aura but no longer controls the enchanted creature
The fact that it has flash means that we can hold it back for either a combat or end-step trick, reserving mana for other things we might need to do
A when you cast triggered ability resolves even if the original spell is countered, and the original spell resolves even if the triggered ability is countered
Your attention is probably automatically drawn to the one going into the man's mouth, but arguably even more disturbing is the maggot boring into his right shoulder, and that horribly swollen, envenomed left arm