Type-1 seesaw is the simplest extension of the Standard Model (SM) that explains light neutrino masses and baryon asymmetry of the universe (BAU) via leptogenesis. In the first part of this talk, I shall discuss how different seesaw models can be visualised graphically and how a fine-tuning in the seesaw formula can be related to a Lorentz boost in the flavour space. I shall discuss a new parametrization of standard Casas-Ibarra orthogonal matrix and show how this new parametrisation is more useful in leptogenesis studies, particularly in SO (10) models. In the second part, I will show that an additional right handed (RH) neutrino could be a DM candidate in the two RH neutrino seesaw model if one considers the new one interacts with the other two with an effective dimension 5 operator induced by SM Higgs. This operator creates a temperature dependent matter potential and therefore the DM (the new RH neutrino) is produced from RH neutrino oscillation due to MSW-like matter effect even if the scale of new physics is post-Planckian. The operator responsible for DM production also causes the DM decay (to light neutrinos) and makes the scenario testable at neutrino telescopes such as IceCube.