While it might sound silly to make your opponent draw a potentially useful card, the steep number of 20 would all but guarantee them milling out and being unable to do anything with whatever they draw
What are the October Card Sets
Heck, I think the surprise is added by the fact that the original Teshar card - Teshar, Ancestor's Apostle - is fairly popular in his own right
For example, if you control Thundermane Dragon (You may cast creature spells with power 4 or greater from the top of your library.) and Twinmaw Stormbrood is on top of your library, you can cast Twinmaw Stormbrood, but not Charring Bite
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A similar thing happens in the third Invaders movie, where you have this reaction after finding out that the Ferroseed you knocked out belongs to one of the fleeing citizens