different between whereafter vs whereat

English

where- +? after

whereafter (not comparable)

(formal) After which time or event.
It is a matter for it to determine whether to make such application, whereafter the Trial Chamber will determine whether the material is so admissible - [1]

where- +? at

whereat

at which, or toward which
- 1793, Samuel Taylor Coleridge, Christabel
  Again she saw that bosom old, / Again she felt that bosom cold, / And drew in her breath with a hissing sound: / Whereat the Knight turned wildly round, / And nothing saw, but his own sweet maid / With eyes upraised, as one that prayed.
because of which; whereupon