The Black Magi are performing some kind of ritual at the druid ring, while the Giantslayers are patrolling the area. Normally, the Black Magi and Giantslayers are found in The Sprawl, and furthermore, at the time this zone was released they were in a three way gang war. Why they traveled to Antonica and are working together here is unknown. Note that this zone was released before LU19 changed The Sprawl, and the fact that The Dreadnaughts took over the Giantslayers is not reflected in this zone. There are no faction hits to worry about.
To start, clear the various kinds of Giantslayers, then clear the Black Magi away from the druid ring. Once you've cleared them all, Black Magi elders spawn.
Finally, once all of the Portal monsters have been killed, a Windstalker grizzly will spawn towards the front on the instance. It is required for completing the quest Attack of the Killer Bear, but has no loot table or unique drops.