Soul Reaver has cast Fire Shield on himself - it will burn anything that attempts to attack him. The spell is defensively reactive - that is, it's hottest in response to offensive, close range action. Unless the tentacles are immune to magical fire they'll be incinerated.
He's also trying to use a Shatter spell to destroy Abaddon's cocoon. The spell basically is designed to break things - the more flawed a thing is, the easier it is forthe caster to break. A normal item would definitely be destroyed with little effort, while a powerful magical artifact would be much more resistant, or even immune. If the obsidian shell has taken a beating already from Gulgrim and Jharm, it'll likely have more flaws and will be more prone to being destroyed.
If it succeeds, an Implosion spell will go off to drive the obsidian shards into Abaddon. Soul Reaver is ready for Abaddon to respond violently to his attack (probably with Drach'nyen) so he's ready to parry with his free hand, but his ultimate goal is to capitalize on the Implosion and skewer Abaddon with Blooddrinker.
