According to the rules, the Overlord deck is reshuffled when you *draw* the last card. However, there are two cases where this wording, as written, could lead to the draw pile being emptied with no way to reshuffle:
1. If the last card in the draw pile is removed from the draw pile without being drawn. This can happen if it is taken due to Schemes or Diabolic Power, for instance.
2. If the Overlord draws all 15 Overlord cards into his hand. Then the discard pile, which is empty, will be "shuffled" into the draw pile. When the Overlord discards cards (e.g. by playing them) they will go into the discard pile, but the draw pile will still be empty, so there will not be another "last card" to draw to trigger the reshuffle.
In this case, would it be correct that the Overlord would not be able to reshuffle the discards (and thus be unable to draw further cards until the end of the scenario)?
An important consideration in answering this question is the following. If it was the case that whenever cards are put into an empty discard pile when the draw deck is empty that the discards are the immediately reshuffled (for instance), there is an infinite loop with Schemes and Diabolic Power. If the Overlord has all 15 Overlord cards in his hand, he could play Diabolic Power (not retrieving anything) and shuffle it back into his deck, then play Schemes to retrieve Diabolic Power, then Diabolic Power to retrieve Schemes, etc. an infinite number of times in one turn. (Schemes and Diabolic Power can both be played at any time during the Overlord turn, and do not trigger off of anything, so the "only one card of the same name per trigger" rule does not stop this.)
On the other hand, Schemes does not reshuffle the Overlord deck by itself if you run out of cards i n the deck while revealing cards, so if the "being stuck with an empty draw pile" scenario was correct than this infinite combo would not be possible.