In episode "A Midsummer Night's Dream" of the Audible version of Neil Gaiman's The Sandman, Morpheus/Dream references a deal he made earlier with Will Shakespeare: Will's stories will live on forever, and in turn Will will write two plays about/for Morpheus. The play A Midsummer Night's Dream is the first of those plays.
Later in the episode,
Dream confides in Tatania that the cost to Will is likely to be greater than the mortal can fathom. Even so, he knows that Will would have taken the deal whether he had known of the cost or not
(quoted from The Sandman Wiki, emphasis mine)
What is this 'cost' for Will that comes with the deal? It does not become clear for me from the context. It might be related to his son Hamnet.
Answers from the original comic book (Sandman Vol 2 #19 (1990)) are welcome.