Maybe these hints will help:
se contulit: the reflexive pronoun here is important; here is what Lewis & Short says about the reflexive use of the verb - "conferre se, to betake or turn one's self anywhere, to go"
cum concilium invenisset: you want the first meaning of concilium here in the dictionary: "a collection of people, an association, gathering, union, meeting, assembly" (this is not consilium, but concilium)
de quibus allatum erat - this is like a parenthetical phrase which you can actually take out of the sentence; it is just remarking on exactly which young men we are talking about here - "the assembly of the young men [[[about whom (de quibus) he had been informed (allatum erat)]]]"
stricto gladio: an ablative absolute in Latin does not have a grammatical relationship to the main subject (that is why it is called an "absolute"), but it often has a logical relationship, and if your goal is to translate into English, often you need to translate that logical relationship for it to make sense:
When he found the gathering of young men, his sword having been drawn... (grammatical)
When he found the gathering of young men, he drew his sword... (logical)
You will find out in the next sentence what he plans to do with that sword!