There's quite an interesting story with EIDOS as it's still somewhat of a mystery as to the purpose of the EIDOS token. Many have speculated that EIDOS is what is known as a "waste token" with the sole purpose of using up the CPU of the mainnet.
Others believe that EIDOS may have been issued to test the limits of the EOS mainnet and it's tx per second.
Still others believed EIDOS was an attack vector designed with the intent to harm the EOS network.
Whatever the case, EIDOS became the crypto-kitties of EOS and clogged up the network, causing major problems for the REX (resource exchange) and most significantly for casual users to make free transactions.
Personally, I haven't been keeping up with EOS much this year for a whole host of different reasons.
This article here is quite good and explains the EIDOS drama and potential pitfalls for similar "waste token" attacks on other DPOS chains.
It's worth reading no matter what your opinion of EOS is.
EIDOS = Everyone Is Denied Of Service