In season 4, Project Diablo 2 introduced a new boss - Rathma. And before releasing the ingredients for the fight, they posted this short verse:

The Ode of Rathma

Hidden from the brightest flame,
The dead remain to speak His name.
Rob the shadow to splinter His light,
turn your soul evil and conquer the night.

Then it turned out that:
"the brightest flame" meant Hellfire Torch
"The dead remain to speak His name" meant Trang-Oul's Jawbone that drops only from undead monsters
"Rob the shadow to splinter His light" meant Splinter of the Void that drops from a certain map event
And 4th line probably refers to the nature of Rathma, he's an evil necromancer. 

Now, in season 10, they're introducing another boss, that we don't know the name of yet. But we know that it's a powerful demon or a fallen angel from Diablo lore, who hasn't been featured in other Diablo games, possibly Lucion, so just keep that identity in mind. The boss might use curses and lightning spells, but it might also not.
We don't know what the ingredients will be, but the speculation so far is:
Annihilus or Annihilus ash (1) + some ingredient that only drops from demons (2) + some shard that drops from a Dark Wanderer map event that summons a random Prime Evil that drops Worldstone Shards, which is prephrased with "Darkness itself wanders these lands..." (3)

The second (2) ingredient should not be an actual item from the game, but some miscellaneous item, like an organ or a piece of a demon's body or an essence. All demons would be able to drop it, so don't make it too specific like a feather or a claw or a scale. It has to be generic, but not too similar to the existing ingredients. And not too grand, since you could get it even from lowly demons. Blood maybe? It can't be ember, because not all demons are burning. Also, please not "essence", it's overused.

The third (3) ingredient could be connected with "darkness" as from the event prephrase.

What I want from you is to come up with the names of the 2nd and 3rd ingredients and write a verse similar to above for each ingredient set. Make it like 5-10 possible verses.

The ingredients and the boss name should not be explicitly mentioned in the verses. Don't mention demons explicitly either. If you don't know what Annihilus is and how to incorporate it into the verses, this might help you:
"The Annihilus is a unique Charm dropped by Uber Diablo.
This item uses the sprite for Mephisto's Soulstone, despite a sprite for a red-colored "Diablo's Soulstone" having existed in the game at the time of creation which could have given it a more unique appearance.
It's like a blue shard. But it can also be red when corrupted. And you can break it into ash."

First 3 lines of each verse should represent each ingredient, and the 4th one is thematic, basically like in The Ode of Rathma.

These ingredients already exist in the game, you can follow the theme, but obviously don't reuse them: Prime Evil Soul, Black Soulstone, Pure Demonic Essence, Splinter of the Void, Trang-Oul's Jawbone, Hellfire Ashes or Hellfire Torch, Tainted Worldstone Shard.
Also, there already are Brain, Horn, and Eye, don't use anything similar.
And I don't like the words "Infernal", "Marrow", "Malevolent", "Abyssal", don't use them for ingredients.

Like in The Ode of Rathma, the first three lines are talking about the boss, follow this pattern. 

Also, as the ingredient, use Cursed Blood (2) and Nocturnal/Midnight/darkness-related Shard/Fragment (3), I think these are best.

For example, I like this verse, but it's not really describing the boss in the first lines and rather giving the instructions:
From the stone of doom's design,
Collect the blood that mirrors thine.
In the realm where darkness dwells,
Confront the foe that terror spells.

I love that it uses "thine" - suggesting that the second ingredient belongs to the same kind as the first one - genius.
And that "darkness" is explicitly mentioned in the third line - clear reference to the map event.
The first line is also cool.

Here's my version:
Break the stone of doom's design,
Draw the blood that mirrors thine.
In the realm where darkness dwells,
Face the foe that terror spells.

Which means breaking down Annihilus to ash, getting Cursed Blood from demons and Darkness Fragment from the map event.
Try to improve it. Make variations of it.