It is actually she who Liddell fights in Artis as the Thunder Lion. When Dorothy is defeated, Thunder Lion's power leaves her and he is then sealed into the portrait with Tinhead and Strawman. Dorothy says to Liddell that all she needs is her servants' power, which she can take from them. This leads to Liddell slapping her and scolding her on how she treats her servants. She then gives Liddell the last Sigil to open the gate to Alice's World and Liddell learns Thun Abyss.
It's still not known, though, how Dorothy could use the rune magic she used on Liddell while she was pretending to be Thunder Lion during the battle. Or how she took the form of Thunder Lion in the first place.