“On to” is similar to “in to”: “on” is an adverb and “to” is a preposition. It often appears in idiomatic and casual expressions:

 

Lastly, “unto” is an old, now rarely used, preposition that can basically be replaced by “to” or “until.”

One of the most famous sentences that uses unto is what is commonly known as the Golden Rule:

 

The Golden Rule uses “unto” in place of “to.”

Here is an example of “unto” in lieu of “until”:

 

“Unto death” (until death) is one of the most common phrases in which “unto” is used.

Source: http://www.theyuniversity.net/post/91583119169/whats-up-with-into-in-to-onto-on-to-and