The poison comming from the snake this rune sprung from is deadly to most creatures. Embedded to your skin, deadly hits can be landed onto your target. That is, if your strike was successfull. The rune lowers your hitting percentage to merely 33% resulting in a lot of misses. Unless you fight enemies with many hit points which are not invulnerable to instant death attacks you might reconsider bringing this rune into battle.
The Viper Rune was introduced in Suikoden 2 along with many other new command runes. In every game the rune shows it's appearance, it is sold in shops and therefore hardly to miss. Suikoden Tactics features both Poison and Instant Death attacks to portion the runes effect into three different commands. Watch out for Lelei and Keneth bringing the Viper Rune into combat by default. - Celes