Visit Anders at his clinic in Darktown or, if Hawke is in a romance with him and invited him to move in, at Hawke's Estate
Received at the beginning of Act 3.
Hawke witnesses Anders either conspiring with Merrill (templars side, rival), refusing service to Isabela on the grounds that he can't/won't treat "that kind of illness", or clashing with Aveline. Anders tells Hawke that Meredith has all but destroyed the mage underground and those left have turned to blood magic as their only option.
As a Rival, choosing the Diplomatic option in the second dialogue prompt ("Maybe it's for the best") results in Anders friendship (+10). Choosing the Aggressive option ("All mages are dangerous") results in Anders rivalry (+10).
If Hawke is in romance with Anders, Hawke can give him a key from the house cellar (that has an exit next to Anders's clinic) to provide him with an escape route in case of danger.