Ted McGinley is the human incarnation of an ancient and powerful immortal being.
Over the millennia, humanity has known Ted McGinley by many names -- The Great Ender, Relief-Bringer, He That Waits Beyond The Shark -- but his earliest recorded appearance is in Norse mythology, where he is referred to simply as Hárogtennur (lit., "hair and teeth"). Though a relatively obscure deity, he plays a particularly important role in Viking lore.
The Vikings were not known for their cultural endeavors; their busy schedule of looting and pillaging left little time for such pursuits. After a particularly hard-fought victory, however, they would sometimes celebrate by engaging in a bawdy ritual dance known as the hjálmurskipti ("helmet swap"). The particulars of this dance are unknown, other than that it involved tossing about the horned headwear favored by the Vikings, drinking copious amounts of beer, and frequent, painful gorings.
The most important aspect of the dance, however, was that once it had begun, the participants could not stop of their own accord for fear of angering the gods (those who had been gored to death rarely adhered to this rule). While initially good fun, after days or even weeks the dance would become tiresome, then excruciating, to participants and observers alike. Eventually, Hárogtennur would appear from nowhere and join the revelers, and this was a sign that their suffering was nearly over. The dance would end, suddenly and with little fanfare, soon afterwards.
Though the Vikings are long gone, Ted McGinley, as he is known to modern folk, has endured. As cultural tastes have changed, his medium has shifted from dance to television, but his role -- to signify the impending death of tedious entertainments and shepherd them gently to their final resting place -- remains unchanged.
It should be noted that although McGinley is frequently referred to as a showkiller, he does not meet the classic definition of that term. While a showkiller will doom any show, regardless of quality or vitality, McGinley usually only brings down programs that are already wounded or simply too mediocre to exist. As a result, McGinley's appearance is generally considered something of a relief to viewers, signifying as it does that the show will soon pass on, leaving room in the schedule to theoretically be filled with something relatively watchable.
Some have speculated that McGinley's powers have waned in recent years. As evidence, they point to Hope and Faith, a show whose doom McGinley's presence has foretold since the pilot episode, but which stubbornly continues to exist after nearly three seasons. Others have pointed out that the demonic powers of Kelly Ripa are likely counteracting McGinley's influence, and that the show is therefore hovering in a sort of limbo, where it shall remain until one or the other relinquishes his grip. It remains to be seen who will win this epic tug-of-war, but it is clear that whatever the final result, the viewers have already lost.