Mordru pre-zero hour showings put him on the lower end of thi tier sans his whole fear of being buried alive. Additionally the whole infinite man scenario was rather impressive too as well as the fact that he will be the last lord of order/chaos standing and he will out last the end of the universe and he is before the universe as well. That said on the whole I don't think he belongs w/ pre-retcon MM nor Jaspers who was multiversal or the Marquis who literally wrecked thousands/unknown ridiculously high number of universes if the number of FF4 teams trying to get at Reed were anything to go by.
An argument could certainly be made that at his peak, Mordru might have reached into the Celestial/low abstract tier. These opponents are all above abstract though.
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"And then there was nothing. A once broken something now void.
And on the first day, Doom spoke... 'Be.'
And then there was life."