Confrontation

Archimedes, older half-brother of Celestia is content to leave ponies to their own devices until he feels the Alicorn linage is being threatened, something he feels is very much the case with young Enigma; son of a race was once mortal enemies to the Alicorns.
It should be noted that Archimedes is the only pony lager than Celestia and possibly the only one she's afraid of. his is due to him being an authoritative presence in her life since she was born and the two have many spats. Celestia doesn't understand why he, who is too wrapped up in his studies to help her with any of Equestrian crises, including the Nightmare Moon incident, feels he has the right and responsibility to interject about her son and eventually her daughter. Enigma is rather timid and though terrified of the huge stallion as a foal eventually grows larger than his mother and while still smaller than Archimedes, it gives him the courage to stand up for himself and his mother. Ths is when Archidemedes realizes how dangerous the foal really is and in that snarl, he sees the face of the one that destroyed the Alicorns.
No it wasn't Discord