Killed some 'immortal' npcs. Frampt has 11,120 HP; the giant bird has 1,160 HP; and Kaathe has 24,464 hp.
Each gives you 5000 souls on death.
See this video for the death of the Everlasting Dragon: http://www.youtube.com/watch?v=O81-6h84LM0
See this video for the death of Seath: http://www.youtube.com/watch?v=ur8w2_gdaIY
Alvina just resets to the default model state on death (open mouth, standing on all fours), drops 1000 souls, has 597 hp, and never disappears. Nito (in the tomb) dies as normal although a little jerky-looking, though doesn't have his "cloak" and is a second entity compared to the boss fought (they aren't the same NPC). He has 4317 hp in the tomb compared to 7080 as a boss fight, but drops nothing. I took videos but don't seem interesting enough to upload
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01:25 Dark Souls - Kill Frampt, Bird, Kaathe