Really need to learn to use spoiler tags, mate. All the spoilers here are VERY spoilerly, especially for anime only watchers, so don't click them if you don't want massive plot points to be spoiled.
Bottom left Shadow is the Orc Disaster. Middle right shadow is most likely the snake he consumed in the cave, I would guess. The arms most likely represent the invading army from the Farmas Kingdom which tried to level Tempest and exterminate its residents and was slaughtered by Rimuru to a man in rage.
As for the flowers, I don't think they mean anything in particular beyond the fact that Rimuru is actually an incredibly dangerous monster when push comes to shove. Looks harmless, but if you judge his threat based on that, you're going to have a bad time.
Those claws, teeth and flesh behind him almost certainly reference the ultimate evolution of his Predator skill, Gluttonous King Beelzebuth
Okay, that name of his ability in the latter spoiler text sounds right out horrifyingly evil.
An elaboration of said last spoiler text:
He also gains Wisdom King Raphiel and Covenant King Uriel. There are seven Ultimate Skills named after the Seven Deadly Sins and seven Ultimate Skills named after the Seven Virtues. They're almost all held by either Demon Lords or Heroes. In theory, if someone could assemble all seven of either you could "unlock" features of the setting's angels or demons, but they're all scattered across Rimuru, his allies and his wards when the dust settles. Ironically, these skills themselves are actually NOT their pinnacle, but the upgrade requirements for their skill synthesis are frankly insane. No wonder: the level beyond this is skills named after Outer Gods.
Ironically, the most "evil" skill is probably Justice King Michael, as it actively seeks to bring out an Apocalypse How by means of unleashing all the legions of angels upon the world.