Rick Stewart/Getty Images

It's rare for a player in the National Football League to enter and instantly become a dominant force on defense, only to fade into obscurity nearly as quickly as he arrived.

Yet that's exactly what happened with outside linebacker Shawne Merriman, who announced his retirement on Tuesday after eight NFL seasons.

The 28-year-old Merriman made it official on his website, according to Michael David Smith of Pro Football Talk.

“After a lengthy discussion with my agent, family and team, I have officially decided to put in my retirement papers today,” Merriman wrote on his personal website. “My retirement from the game I love so much and from the game that has brought me so many opportunities on and off the field has been decided with great thought for my future on and off the field. I retire today not because I don’t feel I can go out there and still play the game at a very high level, I am retiring because I want to retire on my own terms and leave while I know I can still physically play the game.”

When Merriman came into the NFL back in 2005, it appeared that he was set on a path that would one day end with enshrinement in the Pro Football Hall of Fame.

A first-round pick of the San Diego Chargers out of the University of Maryland, Merriman wasted no time making an impact. Despite not cracking the starting the lineup until Week 7, Merriman finished his first NFL season with 57 tackles and 10 sacks, taking home Defensive Rookie of the Year honors.

Merriman picked up right where he left off in 2006, racking up 5.5 sacks in the Chargers' first five games. He also gained fame as "Lights Out", in reference to his high school nickname and the celebratory dance he performed after each sack.

It's at this point that the lights first started to flicker on Merriman's career.

In October of 2006 it was announced that Merriman would be suspended for four games as a result of a violation of the NFL's performance-enhancing drugs policy.

Merriman claimed that the positive test was a result of a tainted nutritional supplement, but sources told ESPN's Chris Mortensen at the time that the cause of Merriman's suspension was something entirely different.

Merriman's positive test was "definitely for steroids … not one of those supplement deals," said a source with knowledge of Merriman's suspension earlier Monday. Both the initial A sample and backup B sample came back positive, Mortensen reported.

Merriman served his suspension, and despite only playing in 12 games in 2006 he paced the National Football League with 17 sacks.

Had he not been suspended that season Merriman all but certainly would have won Defensive Player of the Year honors, and after he once again hit double-digits in 2007 with 12.5 sacks it appeared that his suspension would be an unfortunate footnote in an otherwise stellar career.

Little did anyone know that Merriman would post only six more sacks over the rest of his career.

Before the 2008 season, tests revealed a pair of torn ligaments in Merriman's knee. Merriman at first tried to play through the injury, stating according to the Associated Press via ESPN that "If you give a football player a decision to play, you know, I'm going to play."

However, Merriman made it through only one game before pulling the plug, undergoing surgery and being placed on season-ending injured reserve.

That's a theme that would resound through the rest of Merriman's playing days.

In 2009, Merriman returned to the field, but between the balky knee and a nagging foot injury he clearly wasn't the same player, managing only four sacks in 14 games.

In 2010, after Merriman expressed reservations about his future in San Diego, the Chargers placed a first and third-round tender on him that Merriman signed after a brief holdout. However, injuries once again limited Merriman (this time to his calf and Achilles), and in October the Chargers placed Merriman on injured reserve with a "minor-injury designation."

That meant that Merriman must be released by the team once healthy, and in November of 2010 his career in San Diego ended.

One day after being released by the Chargers, Merriman was claimed by the Buffalo Bills, and at the time general manager Buddy Nix relayed to ESPN the team's excitement at having Merriman in the fold.

"We think Shawne can help our defense," Nix said in a statement released by the team. "He has been an impact player in a 3-4 defense, and we feel this is a good opportunity to make our team better."

That excitement didn't last long.

Merriman aggravated his injured Achilles in his first workout with the Bills in 2010 and didn't play a down before being placed on injured reserve. In 2011, he played in all of five games and registered one sack before the same injury ended his season again.

The Bills released Merriman in August of 2012 but brought him back in October, and in what appears to have been his last NFL season Merriman registered 17 tackles and one sack.

So what happened? How did things go downhill so fast?

Merriman's critics and detractors will simply point to his steroid use and move on. Merriman's success was a result of his juicing, they will say. Once he got caught and stopped using his body broke down and his true talents as a player were revealed.

In fact, as recently as 2011 there was a report that Merriman was detained at the Canadian border for possession of a controlled substance.

However, Merriman vehemently denied those allegations, which were never proven to be anything more than rumor-mongering.

The fact is, we'll never know exactly what (if any) correlation there was between Merriman's alleged steroid use and the injuries that derailed his career.

Yes, he was suspended once for testing positive for a banned substance. However, he also tested clean any number of times before and after, we don't know for a fact that it was steroids he tested positive for, and at any rate you have to ask yourself one question.

If Merriman was juicing, then after his career began its death-spiral wouldn't he have started again, even with the risks that a second positive test would bring?

Mind you, I'm not defending Shawne Merriman. I'm also not going to vilify him.

Because at the end of the day all we know for sure is that the career of a player who was once one of the most feared defensive forces in the National Football League appears to be over at 28.

Whether that's a sad story or a cautionary tale is up for debate.

Follow @IDPManor