tl;dr: Ihsir's Wiles deals extra damage when used on a unit that has burst heals (with multiple healing ticks) or multiple attacks. The extra hits from Ihsir's Wiles will normally perfect spark against the extra hits a unit might get from an ES or SP enhancement.

So Ihsir's Wiles is a silly sphere that does silly things.

Let's break it down:

• Ihsir's Wiles adds an additional attack to the equipped unit. This attack is an ST with a base BB mod of 600%, and 200 flat ATK, which are normally pretty standard values for a third attack.
• The attack has an effect delay of 0 frames, but occurs after all non-SP proc effects, meaning on global units it should normally be buffed by any buffs that aren't part of the unit's SP enhancements (some JP units may prove this wrong as Alim liked to make unit's effect delays increment for every buff, while global units use effect delay 0 for all buffs and effect delay 1 for attacks, generally speaking)
  • This is also important for how the attack sparks, which will be explained in a little while
• The attack forms its hit data from the following:
  • It has one hit matching every hit in the unit's first Attack/Burst Heal effect
  • It has one hit matching every hit in any additional Attack/Heal effect in the unit's core BB/SBB skill set, but removes the last first hit. If the attack/heal only had one hit, Ihsir's Wiles will ignore it entirely.
  • It ignores any extra Attack/Heal effects from ES and SP enhancements
  • If a unit has absolutely no frame data to pull from on the BB or SBB in question (i.e. a pure buff unit that doesn't even burst heal), Ihsir's Wiles adds a single hit attack on frame 0.
  • Random Target attacks count as a single hit because they only have one hit in their frame data and the RT proc repeats it
  • An attack that only triggers against certain elements still contributes all of its frame data, e.g. if Chloe attacks a water unt, Ihsir still borrows the data from her Light/Dark only AoE
• If the hits it pulls from have matching damage frames, they self-spark
• Notably, the extra hits borrow their distribution from the original hits as well. This means if it inherits two attacks' worth of hits, it inherits two attacks' worth of damage distribution.
  • This effectively means the damage is multiplied by the number of procs it draws hit data from, minus the distribution of the dropped hit - for example, Lasswell gains 25 hits, matching 21 from his main AoE and 4 from his ST attack. He doesn't gain a single hit worth 20% distribution from his ST attack, so he deals 180% damage on the Ihsir's Wiles attack. A unit that has a large number of hits on a secondary AoE would normally lose less damage. As this is a true multiplier, it bypasses the ATK cap.
    • The example Lasswell has: An HP-scaled AoE dealing 100% damage, a regular ST dealing 100% damage, and a bonus ST from Ihsir's Wiles dealing 180% damage

Now, for whether this sparks anything:

• The extra hits will spark any matching hits from an effect delay 0 attack they copy. Though rare, this includes a few units such as Randolph & Ravenna.
• As mentioned above, if the extra hits it pulls from multiple procs have the same frames, they self-spark
• Finally, if the unit already has an extra attack from its ES or SP enhancements, Ihsir's Wiles matches that attack exactly for frames and perfect sparks it

For fun, this also adds two attacks' worth of damage to units that have a simultaneous attack and burst heal, such as Alice. Her BB has 44 hits with Wiles and self-sparks 28 of them. It deals damage as if she had a triple attack.

This actually begins to explain an issue we found with units that have extra attacks in their ES starting with Savei; when Laresa came out we thought that the ES inherited the entire attack, and then Savei and Blaze came along and were like "nope, ours don't match". Turns out they use the same logic to work out their extra hits.

So uh, those units with extra attacks on their ES? They all get the damage distribution bonus I mentioned earlier as well (in other words, they've been doing about 33% more damage than they should on paper, but nobody noticed). And their ES attack will always perfect spark with the bonus ST from Ihsir's Wiles...

This also means that Ihsir's extra attack in his SP enhancements will perfect spark if he's equipped with Wiles.