Maybe the symbol behind Harry has something to do with the runes on parts of Demonreach or perhaps it has something to do with Harry being Starborn, seeing as what might be tendrils of energy from the symbol appear to be interacting with Harry’s staff.
However, there is another possibility. Lasciel’s sigil was described by Harry as being vaguely suggestive of an hourglass. The symbol behind Harry fits that description, if the hourglass was sitting on its side.
To back up this possibility, about a year ago Jim dropped that Hannah Ascher was PO’d that Harry had dumped several tons of molten rock on top of her in Hades vault. It seemed to me rather odd that Jim let us know that Hannah is still alive. Especially odd if we weren’t going to see Ascher or Lashiel for several books.
Perhaps the tendrils reaching out towards Harry’s staff mean that Harry isn’t finished dealing with Lashiel in some manner, besides her trying to kill him.