As you know, all can be used with personal pronouns, right?
I want to know what exactly it is; an adverb or something else?
They've eaten it all.
All it takes is 5 minutes to rock the cloud.
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'All' can be a determiner, a predeterminer, a pronoun or an adverb.
For each I'll give an example.
The boys played video games all day.
Have you done all your homework?
All you need is a hammer and some nails.
He got all wet.
In your first example 'all' is used after the object pronoun for emphasis which is a determiner here the other way of saying this is 'all of it'.
I quote this part from Cambridge Dictionaries Online at http://dictionary.cambridge.org/grammar/british-grammar/all
When all refers to a personal pronoun which is the object in a clause, we can use pronoun + all or all of + pronoun. The pronoun is in the object form:
I used to have three pens but I’ve lost them all. (or … but I’ve lost all of them).
Not: … but I lost all them.
However, in short responses, all of must be used:
A: How many of these boxes are you going to need?
B: All of them.
Not: Them all. ×
In the second example of yours it's a pronoun which means 'everything' or here to customize the definition I'd go with 'the only thing'.