The name is a riddle of mine, but it takes a long explanation:
As you know, Jahannam is Arabic for "hell" and Allah is Arabic for "God". Therefore, although western Christians identify "Allah" as the Islamic god, actually Arabian Christians also refer to their God as Allah, and Jesus as the "Son of Allah".
By the same token, Jahannam is hell in the Qur'an, but could also refer to the Christian hell. In other words, words like "Allah" and "Jahannam" which at first seem heathen, are in fact Christian concepts- it is not the word itself, but the way they're used that matters. Take the inverted cross of St. Peter which Christians condemn as "being of Satan"; actually the inverted cross was a Christian symbol for two thousand years, it's just that some Satanists came along and tried to steal it for themselves. Same thing with the rainbow- the rainbow is the symbol of God's promise to Noah, not "gay rights". What's important isn't the word or symbol itself, but how it's used. In my opinion Christians too often judge something as "evil" which is, in fact, good.
That is the first reason for the name.
The second reason is this: the fires of hell do not burn innocent people. Hell-fire only touches the wicked. Hell-fire is the great fear of Satan and demons. Therefore, what at first seems "evil" is, in fact, the truest expression of God's divine justice. Therefore, if the flames of hell were living flames, then that spirit would be the truest expression of God's justice, because it condemns what is evil but does not harm what is good. The Bible also speaks of an intense "fiery trial" that cleans and cleanses, again- another fire which destroys evil, but this fire purifies what is already good and makes it better, like "refined silver" (forget the exact verse).
Thus, "spirit of Jahannam" doesn't mean a spirit dwelling in hell, it means a spirit that is an expression of hell-fire itself.
This isn't to say I'm trying to make myself out as something great and wonderful, just that I approve of the concept. So that's the reason for the username.
As for mystical experiences... Well, experiences in terms of sensing God's presence drawing near. For instance, in one case when I was angry about something, I turned my thoughts to God and said God's name out loud. The instant I did, all the anger disappeared and I felt extremely calm. I was slightly surprised, because intellectually I knew I should be angry, but it was gone, and all that remained was peace. I also sensed God's presence being nearby at that particular moment, and knew that he had done something.
Or, on another occasion, while in prayer, I was filled with an intense joy- I figured out that the joy wasn't natural (ie. from myself), so I wondered: was this joy from God? Or was it a demonic deception? I prayed about it, that God would keep me from all deception, and that if this joy wasn't of him I was willing to give it up immediately, but that if the joy was from him, he would perform his will; but as I prayed I had a sense of joy and peace, as if to say, that the joy was godly. It was as if, that the more willing I was to say, "If this joy is not your will, I will give it up readily", the more joyful God seemed to make me.
On another occasion, I was near someone who blasphemed God very fiercely, and the instant they did, I suddenly sensed the presence of the Holy Spirit, who seemed to recoil in a way that almost made me sick, and later on I sensed the spirit of Satan about that person.