I asked if you can ready an action with someone's reaction as a trigger. The answer is YES.
But I'm still not sure - in such a case (i.e. a trigger for reaction being another reaction) - which reaction is resolved first, the triggered one or the triggering one?
Does the readied action interrupt someone else's reaction that triggered it? Is there a possibility of a chain of such consecutively triggering and consecutively interrupted reactions?
From the rule on the Ready action:
… take your Reaction right after the trigger finishes …
This applies specifically to the Ready action; Reactions that don’t depend on the Ready action (e.g. opportunity attacks, reaction spells) have their own rules.
Some of these, like Counterspell , specifically interrupt the trigger (so, yes, you can Counterspell a Counterspell). Others don’t. And one, Shield, has a weird time-travel effect.
However, for anything hanging off the Ready action, the trigger gets fully resolved first.
The Ready actions defines when the reaction is taken (PHB p. 193; emphasis added):
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.
If your defined trigger is someone else's reaction (or whatever act that reaction includes) you can take your Readied action after that reaction has finished.
Other reactions may actually interrupt, ie. resolve before their trigger, which generally only occurs when the reaction modifies or prevents the trigger, such as counterspell and the Lore Bard's Cutting words (which resolves before the effect resulting from the roll).
The full text on reactions:
Reactions
Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's. The opportunity attack is the most common type of reaction.
When you take a reaction, you can't take another one until the start of your next turn. If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction.
So we can see here that reactions are instant responses to a trigger. Lets say that the trigger is Bob casting the spell Fireball. Alice, Bob's opponent, doesn't want to take a fireball to the face, and promptly Counterspells, so as not to be burnt to a crisp. Since Bob does want her burnt to a crisp, he decides to Counterspell Alice's Counterspell. So now we have this scenario:
In order for the spell Counterspell to work as intended, the most recently used reaction must resolve first. If we had more wizards available, then we could theoretically have a chain of Counterspells as long as the [number of available casters] +1 (though as a DM, I personally would restrict it to only 4 or 5 reactions triggering off each other)
Since Readied Actions and Reactions use the same rules, we can safely say that a similar chain of events can be perform. Bob Readies an action to attack when Alice attacks. Alice readies an action to attack when Tim attacks. Tim readies an action to attack when Jane attacks. We get to Jane's turn, and she takes a swing at Bob, triggering Tim's Readied Action, which in turn triggers Alice's Readied Action, which then triggers Bob's Readied Action. Then we resolve in revers order, Bob hits, then Alice hits, then Tim hits, then, finally, Jane hits.
