[Kameron8]

Nah I'm willing to admit this is a fair balance, Dullahans get their interference resistance, Equipping Mayelia makes it so the summoner has to deal with 15% HP Regen per round when this happens as well, so there is ways around it.

And, this has happened to my Dullahan before too, with the Mayelia, the summoner was not able to out-damage the heals and eventually ran out of FP, so really the summoner boned themself by using it, rather boring, but it still works.

This is nowhere near balanced. Just because Mayelia can potentially save the Dullahan's life in this scenario doesn't level the fact that consistent application of Soul Chain will easily lock the Dullahan in an eternal stun. All this suggests is that Dullahans shouldn't wear any body equipment but Mayelia if they don't want to be instantly doomed, which is completely terrible design for a gimmicky race. (Not to mention that Summoners have the tools to easily outdamage a 15% heal per round)

That being said, I'm in agreement with the OP: Soul Chain (and any other skill that prevents FP regeneration like it) should face the same exception that Interference grants for Dullahans.

Dullahans are supposed to have a 'counter' in that they stun themselves when they exhaust their resources. You get a free turn to try and make up for the fact that they are brutally impassable walls that (almost certainly) have more strength, resilience, and momentum than you. Because of the fringe case where they find themselves permanently stunlocked by interference or soul chains, Dullahans were given a resistance to the former. And now, instead of making the reduction equal to scaled Sanctity, they gain the free 50% in addition to Sanc. Drop a solid 40 points into it, and you have made yourself immune to interference (and to a lesser extent, critical hits).

That means the suits of armor are unstoppable war machines compared to most other races. Wearing Mayelia will not just potentially save the Dullahan's life, it will give them an enormous effective health bonus from healing. While it's counter-intuitive that Grand Summoners can keep Dullahans stunned forever, it's even less intuitive to make these juggernauts stat away their glaring weakness and turn it into a strength.

I apologize that this is slightly off topic; this was mostly about Dullahans as a whole rather than Soul Chains. While I agree it should be subject to the same reduction as interference, the amount of reduction a Dullahan can achieve should be ONLY scaled Sanctity. I'm not even entirely convinced Mayelia should work with their self-inflicted stun.