Treasure Tables??

Apologies for posting so much, I'm just very excited for this session and want to make it as good as possible.

So, I've created the boss, but I don't know what to give for treasure/reward. I know that I would definitely like to give some health potions, but other than that, I'm honestly clueless. For any who missed my previous post, the boss was a Gargantuan undead amalgamate. The setting is deep in a cave system. This amalgamate has killed hundreds or thousands of people over the centuries, mostly soldiers. What kind of treasure could I give for this? They all have magical weapons already (although these magical weapons are, at the moment, weak compared to what most people think of. I didn't want them to be too overpowered at just level four).

So, uhm, ideas?

Because it's killed so many different people over such a long time, you can basically put in whatever you want. You could always just roll on the tables in the DMG for your monster's CR equivalent. That is a limited selection though if you've got a lot of the additional books. An alternative option is to give them lots of money and let go them shopping for what they want (though you'll have to have some reason that more powerful items are not available - either you don't give them that much money or the town(s)/city(s) are too small to have such powerful items).

Some possibilities:

• Soldiers => enhanced weaponry or armour. You could also just leave some better mundane armour/weaponry than the party have.
• Adventurers => corpse slayer if they knew what they were getting into. Flame Tongue Weapon, a bag of holding (a favourite of adventurers, great utility but not OP). A few potions other than healing. I'd recommend just look at the list of rare potions you've available and pick two or three.
• Miscellaneous -> an eversmoking battle, deck of illusions, pipe of remembrance or any other fun item that has specific utility but isn't OP.
• The people it's killed have been mostly poor weaklings, they didn't have magic items but they've left you a small fortune in money or expensive items.
• Acid/poison from this horrifying monstrosity - as long as they have ~ to carry a bit of it away.

Others:

• If you've access to Eberron, Rising from the Last War, Kyrzin's Ooze might be a thematically fitting item, but you might find that a bit strong (though the curse could be nasty).
• If this is a particularly significant battle, you could offer the party some of the lessor items they've been asking for or that you know would benefit their character.
• Homebrew some kind of evolution item(s) that will grow with the players, so it'll never be stronger than you meant it to be (if you homebrew, always put in the caveat that you're trying this out and if it's too strong you will adapt it)

Generally you've left it very open so (and I'm sorry, I'm aware this is not the most helpful advice), you can make the hoard contain what you want. Because of their level, personally, I'd limit it to a few magic items at most and mostly give them money. 

Hope this helps.