It's been a long time since it's been used, so as a reminder: the Spear of the Apocalypse is a projectile similar to Soul Reaver's Sphere of Annihilation. However, whereas the Sphere of Annihilation explodes into a huge sphere that tends to vaporize anything within it, the Spear of Apocalypse is more restrained, its energy remaining focused largely on and immediately around anything the Spear actually strikes (which makes it slightly easier, faster, and less draining to cast - but it's still a very high-level offensive spell).
It'll travel in a straight line and cause massive damage to anything that it hits without stopping. Soul Reaver judges, by the size of the crystal, that it'll either explode it into a million tiny bits or, if its structure isn't exploded by the extreme pressure/temperature differentials of the impact, will obliterate near half of its mass in a nice tunnel-shaped hole through the middle.