Despair could enter memories simply by looking at someone or something.
In those memories he could play as he wished, crushing, altering, and manipulating them to his liking.
However, there was also the condition that the person must have a destroyed will for Despair to enter, and he could only enter their 'bad' memories.
Someone like Ixion was nearly immune.
In any case…
Through grasping and injecting Aether into one of the metal rods, Despair could theoretically draw another person into a memory.
A memory of a time of complete loss in that host's life, when their mind had experienced the most pain and suffering imaginable, mental or physical.
'How did Hope not realize his brother was wicked with an ability like that…? Who knows?!'
The condition to be drawn into one of these memories alongside him was simply the need for a physical body and Despair to allow it.
'Not only is his hiding space invite-only, but he could be in any one of them!'
