How would you introduce a new story arc in the middle of another arc? Let's say that you stopped writing a comics series for 3 years and the last time you left off is in the beginning of a new arc, but let's say because of your health you need to end the story. How would you introduce the last arc of the story in the middle of the beginning of a new arc? Are there techniques used that can accomplish this?
If you're trying to suddenly end an arc and start another, the best way to do it is most likely this.
The Protagonists Are Out of Time. The Gauntlet's Been Thrown. Last Arc Was a Side Quest. There's No Time For Those Anymore
It sounds like you left your story off on something of a cliffhanger and are now coming back to it only to find you wish to wrap it all up as soon as possible.
Starting a new arc in the middle of another one is possible, but it would be easier to simply complete what the characters were doing in the first arc and then jump into the final arc.
For example, perhaps the main goal of the story was that the characters were all going to slay the Demon King. When you left the story off last, the characters were all on a quest for the Holy Grail. You went on a writing break right before the characters were about to find the Grail, so now that you're starting over, you want to introduce a new plot thread.
The worst way you could handle this is shifting gears so abruptly that it gives the audience whiplash. Halfway through the adventure for this ancient artifact the characters all get distracted, see a butterfly, and completely abandon the quest they were so excited for only a few chapters ago.
Forget the Holy Grail, we need to visit this new town. Or explore these haunted ruins. Or go to a festival. Fight an army, slay some undead, battle some giants, etc.
You don't want to ruin your plot progression by having the characters go from doing X, never finishing it, and immediately going to Y.
Unless, X was entirely pointless in the first place or something else takes precedent.
Example. Your characters wanted to find the Holy Grail. They think it's the only way they can beat the Demon King. Surprise! It was a trap. The Grail's not real. Or the Demon King already destroyed it because he knew it was his only weakness. He was lying in wait for the heroes and now they have to fight him to the death. Cue final battle.
The only reason to cut an arc off abruptly is for something else to take precedence. The original quest was a wild goose hunt so they have to shift gears. It could have been a trap or a trick or just bad luck. Maybe the quest was important but they've already run out of time. Doomsday's already here.
The characters wanted to have a fun training arc to prepare for the bad guy. Too late. He's already on your front door. That's what the heroes get for thinking the villain would wait for them to strike first.
There's no more time for comedy. There's no more time for hesitation. The plot burst through the wall and all the chips are down.
If the side quest is vital to the main plot and the character's can't skip it, just add new stakes to the battle.
When they were first going on the quest for the Holy Grail everyone thought it would be a simple fetch quest but this was actually the place where the final battle would be taking place. You're still finishing the arc, but it's been recontextualized so now the action and drama has been ramped up.