Waves for Epic Combat
If you want to give your players a different, way more intense combat experience, or if you want your next session to feel like an all out war, consider using waves of enemies. A lot of DnD encounters are discreet events, often beginning on the players terms and providing opportunity to recuperate afterwards. Most abilities and spells with a one minute duration might as well be infinite. But by cutting away those spaces between a string of encounters it's easy to hit that ten round mark and keep the tension escalating instead of falling away.
You don't want to do this every time of course because it is going to take a toll on your players, let alone their characters, and it is also going to be harder on you the DM. But if you want the boss fight to live up to expectations after being built up for weeks, maybe months, have them send out rounds of minions before they join the fray themselves. This is excellent if things are playing out in a large space, with the hobgoblin king throwing commands and taunts from just outside of range or behind cover, so the players know to hold back a bit as they clear this wave of goblins that are on them and the next batch that are running in.
I could also see something like this playing out in tighter quarters as the evil arch mage tactically retreats from the party, leading them through rooms of underlings. Actually, this is a great way to give your big bad time in the spotlight before things come to blows. The party could come upon them at the entrance of their lair and be led through a maze of traps and other enemies before finally cornering and confronting their quarry.
Using waves of enemies also gives you a degree of control. Of course you can leave it to fate and add d4 blights every round until they defeat the Gulthias Tree, but you can also keep your feet on the pedals and pump the breaks if the party has a bad round, or step on the gas if they clear the latest wave quickly. This makes it a great tool when things go sideways and they've alerted the entire dungeon that they're here. If you planned for them to clear out this cave network over the course of a few sessions and the bard just cast thunderwave, and now everything in 300 feet (before we even consider echoes and intelligent beings communicating with each other) is going to come running. If they are going to TPK if they fight everything at once, have the baddies arrive in waves, give the party a way out and make it crystal clear that there are a lot of enemies coming and maybe even more on the way.
And with this tempo adjustment you can create that perfect little cinematic, eye of the storm moment when the final minion drops and the big bad finishes their monologue, steps out of the shadows, cracks their neck and casts mage armor or haste on themselves. Especially if there is some distance this gives the party a round to, buff, heal, and get it together a little bit after a long slog against the initial waves, and maybe even say something cool. And if the big bad is almost smoke after the first round, well, here comes another wave of minions to help them out at top of the next one.
