If your nonlegendary Blightsteel Colossus is given myriad by Cybermen Squadron, when it attacks it will create tapped attacking token copies for each other opponent (the myriad effect), and those tokens enter the battlefield as copies of the Colossus . Those tokens have infect, so damage they deal is infect damage (and is still damage in all respects) [2020-08-07, wotc] and can give opponents poison counters; a player with ten or more poison counters loses the game . If a token copy would be put into a graveyard (or any zone other than the battlefield), the token ceases to exist and you do not get a card shuffled into your library from that token; the original (non-token) Blightsteel Colossus’s own replacement-to-shuffle ability still functions for the real permanent, but it cannot save token copies because tokens cease to exist in non-battlefield zones .
Step 1
You declare Blightsteel Colossus as an attacker. Cybermen Squadron grants myriad to your nonlegendary artifact creature, so the Colossus has myriad (Cybermen Squadron’s oracle text).
Step 2
When Colossus attacks, its myriad ability triggers. For each opponent other than the defending player you may create a token copy that's tapped and attacking that player or a planeswalker they control; multiple instances of myriad (if they exist) would trigger separately (Ruling [2023-10-13]).
Step 3
Each token created enters the battlefield as a copy of Blightsteel Colossus, so they enter as copies and gain the Colossus’s characteristics (including infect and indestructible) as they enter .
Step 4
Those token copies are tapped and attacking the appropriate opponents or planeswalkers as specified by myriad’s effect (Cybermen Squadron’s oracle text).
Step 5
If a token copy deals combat damage to a player, that damage is infect damage and is damage in all respects, so it gives that player poison counters equal to the damage dealt and triggers damage-based abilities normally [2020-08-07, wotc].
Step 6
If a token copy would be put into a graveyard or otherwise would change zones to a non-battlefield zone, it ceases to exist instead of existing in that zone, because tokens can’t exist in zones other than the battlefield . As a result, you do not get a physical card to shuffle into your library from a token copy’s 'if Blightsteel Colossus would be put into a graveyard…shuffle it into its owner's library' line; that replacement can’t produce a lasting card in a non-battlefield zone for a token.
Step 7
If the original (non-token) Blightsteel Colossus would be put into a graveyard, its printed replacement effect still applies to that real permanent and you would reveal it and shuffle it into its owner’s library instead of it going to the graveyard (the token limitation above does not prevent that for the real permanent).
Step 8
Any tokens created by myriad are exiled at end of combat as part of the myriad templated text (Cybermen Squadron’s oracle text).
Resolution
When your Blightsteel Colossus attacks with myriad from Cybermen Squadron, you get a separate token copy tapped and attacking each other opponent (per the myriad text on Cybermen Squadron), and those tokens enter as copies of Blightsteel Colossus . The token copies have infect, so damage they deal is infect damage (damage in all respects) and will give poison counters to players as normal [2020-08-07, wotc]; if a player reaches ten or more poison counters they lose the game . If a token copy would be put into a graveyard or otherwise end up in a zone other than the battlefield, it simply ceases to exist (tokens can’t exist in those zones), so you don’t get the shuffle-into-library effect from the Colossus’s printed replacement ability for those tokens . The printed replacement ability to shuffle instead of going to the graveyard will still apply to the non-token Blightsteel Colossus if that actual permanent would be put into a graveyard.
