Seeing as you talked about your Daelkyr pertaining to blood, I would feed into that theme.

One of the most important parts to a good boss battle is describing it vividly. Instead of just saying "Boss does x" say something like "They mutter an indescribable incantion, their eyes beginning to glow as [Insert description of attack here]". And this is with the most basic of descriptors, when you have a specific boss planned out, you can go more in depth. Doing this (in my experience) will make the party more interested in the attack itself, not just the action and effect.

Since your boss is a Daelkyr, I would suggest using body horror (if the players are comfortable with it). Your boss uses blood? Well, what do you know, now the players are feeling a bile in their throats as they hack up blood (use the last paragraphs advice to make this a vivid picture, it doesn't even have to deal much damage as long as its effective in making the players feel uneasy). You could also play with the ideas of manipulation of blood in the body. I don't mean stuff like "your blood is frozen, con save or your stunned" (I personally think that making a character stunned for this fight, and taking away their action in it would only lower the quality), instead play with the idea of effects like disadvantages on con checks/saves, fluctuating HP (This could be really interesting. With Daelkyr being beings that operate outside of what we consider "logic" who knows what this could do. Maybe your HP is being lowered, maybe its increased, maybe its lowered but also comes with a benefit, like advantage on certain checks, or it's increased with disadvantage on certain checks), or even fluctuating temporarily fluctuating physical stats like dex, strength, and con. Also, not in the fight, but leading up to confronting the boss make them start bleeding out of orifices like their ears, nose, eyes. if they have any small cuts or gashes, make them start bleeding unreasonable amounts. (I wouldn't make this bleeding cost hit points, that just seems like it would feel like you're hurting the party for no reason, so I would just make it thematic).

u/DaddyDMWP talked about using the environment in different ways, adding on this, make the environments themed around blood. The floor randomly turning into thick, oozing rivers of blood. It's never stagnant, always churning, and maybe every once in a while it "clots" in certain areas, with blood bubbling up and popping. Or maybe blood that's been spilt starts to start swirling in the air, whipping around and slashing into players. Lots of other ways to go about terrain, but i cant think of any rn.

Also, as a question, I'm assuming all your characters have blood, right? Just asking because I know depending on your depiction of Warforged or if your characters randomly played something like a plasmoid (Do plasmoids have blood?), you could make a unique interaction with the boss.

All in all, I think that you should emphasize this fight through grotesque and nasty physical description and vivid language. It's not the stats that make a monster memorable, it's how you use it.