Mysterium Tremendum et Fascinans was a phrase coined by R. Otto to refer to something one finds to be absolutely frightening yet awe-inspiring at the same time, something one feels helplessly drawn to yet wants to violently flee from in the same instant. Broken down is basically this:
mysterium - the 'wholly other' (Otto) that which is totally transcendent and apart, although is open to some degree to our experience
tremendum - the terror, awfulness, dread, overpowering, majesty, of God. Often linked with this is Otto's 'Creature Consciousness 'theory, (which I think is probably right in a sound context): a feeling of absolute 'nothingness' in comparison to and approach of God, or - , a feeling of total depravity and dependence upon God
fascinans - an attractiveness, wonder, obsessive interestedness in spite of fear
One may link this to Ezekiel's, Isaiah's, or John's revelation visions. The feeling of approaching the throne of our saviour - do we hope for it daily? Do we usher it in by our prayers? I find sometimes when I pray or meditate, this feeling of fear and awe, this knowledge of God's awesome wrath and his Fatherly love, this mysterium tremendum et fascinans, and it is such a wonderful dynamic of God's character, our place in light of Him, our appropriate response to Him, and our relationship with Him.
(some readers who have read Otto or Ninian Smart may recognise that I haven't entirely used this concept in the original context but linked it to Biblical and Christian experience.)