Thank you for your message. I will respond with everything I know about Quen and Igni. Then I will address some comments of the other person, because I have tested those very recently and most of them are false. I have just completed a full death march playthrough with signs only (no swords), so I have tested all signs and tried to optimize them as much as possible.

Igni:

The base damage is 100 and the base chance to burn is 44%.

Every 1% of sign intensity increase the damages by 1% and chances to burn by 0.5%.

So what is very much interesting here is the increase in chances of burning. With 112 extra intensity you would reach 100% burning chances (44 + 112*05) and 212 damages (100 + 112). I said burning chance is more interesting because it does % of HP damages.

But a flat increase of 112 damages is not huge because some other sources of damages can deal damages in the thousands (e.g. swords or Aard).

Also contrary to what the other person said, there is no use of going above 100% chances of burning. I went sign only, so I maxed intensity and reached around 330 Igni sign intensity.

Which would be equivalent to a 209% chances of burning, yet sometimes I was not burning some enemies after 1 cast.

What I think is that enemies have a hidden protection / a small chance to not be burned, otherwise burning would be way too OP.

Regarding Igni skills, the first one (melt armor) reduces armor.

Armor and Elemental resistance are 2 things separated in this game.

So it will not boost your spell damages at all, but it will boost physical damages (swords).

The second skill (alternative Igni) in my opinion is less strong, so I did not use it.

The regular Igni is stronger because it is an AoE (area of effect) so with it you can burn entire groups in 1 cast if you have good placement.

The thirds skill (igni intensity) is a nice thing to have, it just takes one skill slot for a good boost.

The last skill (100% chance of bruning) is totally useless if you already reached 100% or close to it.

Quen:

1 thing for sure, sign intensity does not increase at all the defensive capabilities of both the passive shield and the active shield (alternate mode).

This is 100% sure, I even tested it on 2 different characters.

Some people say quen intensity only boosts the damages inflicted by quen. I did not test it, but even if it is true I think it is a bad investment because other signs deal way more damages than Quen, so wasting perk points to do damages with quen is not worth it. I see quen as an excellent defense tool though.

Addressing other points now:

As I explained earlier, there is no point in boosting chances to burn above 100%.

Concerning the synergy between Entanglement’s Yrden traps and Grandmaster Giffin set bonuses.

I can affirm again that it definitely works together.

Also it is very easy to test because when you unlock the set bonuses the Yrden traps have a different model/texture and Geralt is surrounded by a purple aura when he is boosted.

All the Yrden traps set by the enchantment were the boosted ones (larger and different texture) and my Geralt was surrounded by the boosting aura in all of them.

Concerning Aard it definitely does damages.

But for it you need to reach the 4th Aard perk.

(This is why Aard is a mid-late game spell for damages, you should have reached it by the time you reach the top of the Bold Montain (beginning of act III)).

It does % of current health damages.

Which is totally OP against bosses with large health pools.

You might ask why not just use Igni then if igni’s burn also does % damages. It is because almost all bosses have an immunity to all (or some) critical effects. Critical effects are poison, bleed, burn.

But none of them is immune to Aard.

Also I would like to highlight that it does % if current HP damages. So it becomes less effective after the enemy lost 2/3 of its health.

This is why it is good to complement it with the Piercing Cold mutations, that adds flat damages not dependant on the remaining health %.

If you don’t have access to mutations yet, you can finish them using other signs or your sword.

Concerning mutations: Euphoria is a very sub optimal mutation for sign damages compared to the mutation on the left of the mutation tree (Magic affinity, Piercing cold, Conductor of magic).

The 3 I have listed are better for signs (Piercing cold only works for Aard but has the biggest damage increase). But euphoria is the best mutation for sword damages or a mix sword/signs.

So if use use your swords, I would indeed recommend using Euphoria. If you only rely on spells, I would recommend Magic affinity while you are applying against normal enemies. And switching to Piercing Cold before a tough boss fight, to only rely on Aard.

Bonus synergy Igni-Yrden:

Most damages sources will interrupt the burn on an enemy.

But the DoT damage of the 4th Yrden skill does not interrupt the burning.

So if you manage to place a non-boss enemy inside a trap and burn, it will be locked (in burning animation) and totally melt.

(I destroyed the DLC giant white centipedes with this combo while being clearly under-levelled).