It goes back to 1873 and Walter Bagehot, who wrote that “to stem a financial panic, [...] the central bank should lend freely at a penalty rate to solvent institutions against good collateral”. The ECB has done that. It has offered the banks unlimited liquidity and extended the maturities of the relevant loans.
At the same time, it has expanded the list of eligible collateral. As a lender of last resort the ECB managed to help the banking system get through the first phase of the crisis. When the crisis started, the ECB also reduced interest rates. Initially, this wasn’t a new path either, but standard practice for monetary policy.