Foreboding (1).

The Summon skill is one of the most mysterious skills in the universe, as it gives its user complete control over other living beings. 

It can be a branch of the nature element, which is so unique, that it can conveniently fuse with any other element to birth an uncountable number of paraelementals. Or, it could be because of something even more mystical...

The creature he summoned, {White Shadow}, was a unique summoned demon wolf whose special abilities were centered around stealth, with very few combat abilities.

At the start of the apocalypse, the special skill he had gained upon killing a devil was the {Demon Wolf Summon} skill series.

What made this skill so special, is the fact that the skill had a special property that allowed him to acquire special summoning spells the stronger he became. At every level of the skill, a new branch of the skill tree would be opened up to him, allowing him to choose which one he would prefer.