The God Man
That which was not assumed is not healed; but that which is united to God is saved — St. Gregory the Theologian (also known as Gregory of Nazianzus)
This beautifully succinct statement by St. Gregory sums up the dogmatic necessity for understanding Christ to be both God and Man. If He is not God, then we have no means to be healed. If He is not Man, then we have not been healed.
Thus, in a fantasy setting, the Christ figure must emulate perfect divinity and perfect humanity. This means that He cannot become a God either through His excellence or by God descending upon Him. This scenario does not assume the totality of human nature — it ignores conception, growth in the womb and childhood. Therefore, all of these things would be left out of Christ's salvific activity. All of our experience and existence must be assumed in order to save it. This includes death.
Psalm 22, written some 300 years prior to the invention of crucifixion, describes in detail Christ on the cross. Christ tells His disciples multiple times that He must be turned over to the Gentiles to be killed. In iconography, the Nativity depicts the manger as a deep, dark cave; His bed as a tomb; and His swaddling clothes as a burial garment. In other words, the whole point of the Incarnation is the crucifixion — the Christ must die in order to destroy death.
In terms of a fantasy world, we must understand the metaphor of the cross:
The message of the cross is folly to those who are on the way to ruin, but for those of us on the road to salvation it is the power of God. — 1 Corinthians 1:18
Crucifixion is a heinous means of killing someone. It is designed to torture and kill slowly over several days. The amount of stress it puts on the body is extreme and it takes advantage of our own instinctual desire to survive in order to prolong agony. It was a death sentence reserved for the lowest of the low — outsiders and criminals. The Romans wouldn't dream of subjecting a Roman citizen to such a death because it was too horrible and too humiliating even for someone who betrayed the Empire.
God, in the person of Jesus Christ, was not only willing to subject Himself to such humiliation and horror, but transformed the instrument of death and torture into a symbol of everlasting life.
Thus, the cross need not be a cross in terms of a fantasy world. In fact, I would argue that we have lost the sense of divine irony that is the cross — we no longer see it as a device of extreme torture — and that using another device would actually more effectively communicate the true meaning of the cross. The holy symbol of those who follow the Christ is the instrument of torture used to kill the Christ. For example, in the Chronicles of Narnia, Aslan is shaved, tied down upon a rock and killed by a knife. The holy symbol could be any combination of these instruments — the knife, the rock, or the ropes.
Of course the ultimate purpose of the crucifixion is the resurrection, the ascension into heaven and the enthronement at the right hand of the Father. Our human nature, in the person of Jesus Christ, is participating in the divine nature of God right now.
In terms of a fantasy world, this is the mechanism by which divine magic works. Clerics join themselves to the Body of Christ — the Church — and through their ordination have direct access to the power of God. Divine magic is a metaphor for the miracles of God worked through the people of God. The pseudo-Christian overtones of Cleric spells in 0e represent this quite well. It is all made possible because the Christ died and then took our humanity with Him to sit in glory and the right hand of the Father — in our humanity, we have access to His divinity.