In your example, the Qu' (for que) points to whatever is happening to Lucas, and the il is impersonal. Like in the statement It rains. It here isn’t really something or someone.

So just as you could say and ask the followings :

It rains.

Does it rain?

You can say and ask :

Il arrive quelque chose à Lucas.

Qu’arrive-il à Lucas?

Careful though! In other situations, il in a question would be a personal pronoun. Like in :

Lucas est à l’école.

Où est-il?

And in that case il replaces Lucas and it would be redundant to ask :

Où est-il, Lucas?

To conclude, some sentences use impersonal pronouns, and if you flip those sentences into questions, you will find again the impersonal pronoun. The more you read and speak French, the more you will be able to recognise them! For a start, here are a few examples :

Il arrive que..

Il pleut, Il neige, Il vente…

Il faut…

Il est tard, Il est tôt…