Whenever Arcturus feels as if someone is in danger, he forms an ice blockade.
3 times in a row, this has happened;
1. Formed a blockade around a full health Serafine with all 5 heroes alive. She lost the chance of using her ult, and my frontline got dropped.
2. None got an ice blockade at all.
3. Same as 2
It even states that he will cast on himself, if hes the last. This has not occured a single time. I have used him well over 80 times.
That's weird, I've seen him cast it plenty of times. He always casts it on himself when I attack him with my Tanya. The only time this doesn't happen is when he isn't below the threshold to cast it yet, and then he receives a big crit which kills him instantly.
The trigger to cast it on himself is not being the last btw, it's if he drops below a certain HP threshold. Basically the first hero to drop below this threshold will get the blockade.