As a general rule, if a unit is both prevented from making, and guaranteed to make, a follow-up attack, the effects cancel out, and the standard speed check decides instead.
A foe with skills such as Null Follow-Up 3 will disable Daggerbreaker's guaranteed follow-up attack effect.
Unit 1
Unit 2
Results
Guaranteed follow-up attack
Prevented from making a follow-up attack
Guaranteed follow-up attack
Prevented from making a follow-up attack
All effects of both of the skills will cancel each other out; a standard speed check (≥5 Spd) determines follow-up attacks for both units. For an example, this scenario can be achieved by having two sword users use Swordbreaker 3 against each other.
Guaranteed follow-up attack
Prevented from making a follow-up attack
Prevented from making a follow-up attack
Prevented from making a follow-up attack
Unit 2 will be unable to perform a follow-up attack on unit 1. A standard speed check (≥5 Spd) determines follow-up attacks for Unit 1. An example of this scenario is a unit using Lancebreaker 3 against a lance user with Wary Fighter 3.
Guaranteed follow-up attack
Guaranteed follow-up attack
Prevented from making a follow-up attack
Unit 1 will perform a follow-up attack regardless of speed. A standard speed check (≥5 Spd) determines follow-up attacks for Unit 2. An example of this scenario is a unit using Axebreaker 3 against an axe user with Quick Riposte 3.
Guaranteed follow-up attack
Guaranteed follow-up attack
Guaranteed follow-up attack
Prevented from making a follow-up attack
Unit 1 will perform a follow-up attack regardless of speed. Unit 2 will perform a follow-up attack regardless of speed. An example of this scenario occurring would be a unit using R Tomebreaker 3 against a red tome user using two Quick Riposte 3 skills.
These interactions can continue for any number of effects that prevent or guarantee follow-up attacks. The way contradictory effects cancel out can be used to understand any number of these effects interacting with each other.
Note that remaining conditions and effects are still active, so Special cooldown charge would still be in play for a Bold Fighter 3 against Wary Fighter 3 interaction regardless of the status of follow-up attacks on each unit.
The skill first appeared in Fire Emblem Fates, under the name Shurikenbreaker in international languages. Since Heroes uses daggers alone the skill is called Daggerbreaker.