One of the purged legions is spoken of as a tragedy. The other has been hinted to be traitorous. So what are the chances of the Legion of the Damned being an actual legion rather than a chapter?

Both Primarchs were found and lost. One does seem to have turned traitor and have been put down by the Space Wolves. Both were considered tragedies and the Primarchs took an oath of silence to never speak of their fallen brothers. The remainders of the legions were folded into the Ultramarines.

That's not really confirmed. The source isn't exactly reliable.

And the Legion of the Damned didn't show up for quite a while.

The Legion of the Damned is probably the Fire Hawks. You can check it out at the Lexicanum.

http://www.blacklibrary.com/games-workshop-digital-editions/Codex-Legion-of-the-Damned.html

in the last codex the lore of the legion of the damned is changed: they can't be more the fire hawk.

or better the fire hawk are not the only source of damned legionaire.

because the legion ia appeared before the fire hawk disappeareance.

now the legion is a sorta of "imperial demon" like the living saints
of the sisters of battle.

the legion appear when and where there is a great danger for space marine or imperial citizen.
their bolter fire misterious warp bolt.
when they "die" they disappear in a pillar of fire.

their true nature is not clear but the inquisition not really hunt them for the reason that they don't appear for hurt the imperium but only help.

1. Novel "Ahriman: Sorcerer": Time-travel to the past is possible because Warp.

2. Talon of Horus: Time-travel is possible because Warp.

3. The Imperium had time-cops to stop time-travellers from meddling with the past, the Ordo Chronos.
http://wh40k.lexicanum.com/wiki/Ordo_Chronos


There is more, but I don't want to post spoilers.

yes is one of the possibility but if you read the codex legion of the damned you understand tha the legion is no more considered to be a contaged by a plague fire hawk chapter. that is relegated as only "one of the rumor", and is basically no more an option that the legion is still living marine that travel around the warp.

just read the new legion codex.

Who cares they're flaming skeletons and I want to play one so bad.

THEY BETTER BE FLAMING SKELETONS. And not just "dudes in armor with flames painted on."

I dont think that they got tainted by Chaos. When Horus had a vision given to him by chaos, Horus smashed the chamber of one of the lost primarchs. I think they betrayed and conspired with xeno. Russ and the Emperor killed the 2 primarchs and the loyalist remaining put into the ultramarines.

"When Dante asks the Blood Angels' successor chapters for men to bring the Blood Angels back to full strength, having suffered heavy casualties during Arkio's heresy, Seth, chapter master of the Flesh Tearers, makes a counter to this demand: disband the Blood Angels and spread them amongst the successors. When Orloc (chapter master of the Blood Drinkers) protests that there is no way a First Founding Chapter can end like this, Seth retorts that, actually, it has happened before"

When the Rune Priest was asked what Leman Russ's wyrd was, the priest stated simply "He is the Emperor's executioner"

It's implied only one legion was truly traitorous. The other being more of a victim of circumstance but still guilty in some way. Just not a traitor.