Sigil of Malice (Leblanc) procs off of Thunderlord's Decree
I'm not sure that it is intended for Leblanc to be able to cast Sigil of Malice, Mimi Sigil of Malice and have the mimicked Sigil immediately get proc'ed by Thunderlord's decree. It kind of removes the whole skill mechanic of trying to land Illusory Chains.
Maybe Sigil of Malice's secondary effect should proc off of 'Spells' as opposed to one of 'Leblanc's abilities that have caused damage'. Or maybe Thunderlord's should be reclassified as an on-hit effect like all the other masteries (which, I'm sure, is why Deathfire Touch doesn't proc Sigil of Malice, which would be even more broken).
I'm pretty sure this is one of the reasons why DFG was removed as an item.