Cruel Ultimatum

[Tobi]

What happens if I play Cruel Ultimatum with me having no cards in my library (or less than 3) and my opponent being at 5 life or less?
Who dies first? 

[Clariax]

Losing to 0 or less life and losing to trying to draw from an empty library are both state-based effects.  Whenever state-based effects are checked, all applicable effects resolve simultaneously, so both players lose at the same time.  And, when both players lose at the same time the game is a draw.