I have Elder Deep-Fiend, a creature which has flash and allows me to sacrifice another creature towards the cost of its mana. When the creature is played, it allows me to tap 4 target permanents.
I have been doing this during the upkeep step/beginning phase of my friend's turn to tap 4 of their lands so they can't use them. However, they say they have priority and add mana to their mana pool before I can.
As I am playing an instant creature which has an ability when it enters the battlefield, just after they have untapped, I think they shouldn't be able to tap their lands first as they are not up to that phase yet.
Basically, they are saying I can only use it to tap creatures before their combat phase.
Who is right?