Assistance Needed

I'm running my first DnD session, both as DM and in general, in the New Year with two friends and my Mrs and I was looking for some help regarding in what would be a good idea for an end boss style monster?

I'm trying to keep the campaign low level and interesting enough to keep everyone intrigued and willing to play more without burdening them too much with indepth lore as its everyone's first time.

Any questions or further info needed, just ask. I'm new to this subreddit as well eheh -awkward shuffle away-