The 'lion' actually is an alien deity named 'Twilight' who wanted the new Universe to be created so it can live. At the same time the lion is actually THE Universe, sort of like Earth is our 'Universe'. Personification-like. That was a s much as I was able to get out of Scott Allie anyhow. After the Twilight plane was opened the lion-Twilight deity stated that it is not enough and that his spanky new 'verse' needs a 'soul' aka 'the Seed of Wonder - aka the shiny red egg thingy that sits under the Sunnydale and is guarded by Master. Makes sense to you? Sure. In order to get to the 'egg' Twilight deity possessed Angel bodily completely, so he could fight Buffy as equal. Why could it not do so in person I am not sure - maybe it could not approach the 'egg' thing without possessing Angel. In any case, through issues 38-39 Angel is not himself and is totally under Twi's control - until Buffy breaks the red egg and closes the conduit to the Twilight plane. Then the lion departs and Angel wakes up to discover he just killed Giles.
Lots and lots of angst attached.